Recrystallization A Purification Technique Project

Recrystallization•
•
Principles
Steps & Techniques
1) Finding a Good Solvent
2) Dissolving the sample
3) Hot filtration
4) Cooling the hot filtrate
5) Separating the crystallized product from mother
liquor
6) Washing the filtered product & drying
•
Checking purity by m.p. test & efficiency by mass
recovery (weight, yield)
Recrystallization – The Principles
Generally, solubility of an organic covalent
compound increases as temperature of the
solvent increases
Solubility (g/1000 mL)
Solubility of Benzoic Acid in H2O
Solubility (g/100 mL)
o
T = (20 C)
0.29
o
(95 C)
6.80
o
Temperature ( C)
Therefore if you had 1.0 g of benzoic acid,
you would need:
(1.00g X (100mL/6.8g) = 14.7 mL
If you don’t know what your compound is,
you go slowly!!!
1. Selection of Solvent
Guideline
• Rule of thumb – “like dissolves like”
o
• Polar compounds (alcohols, carboxylic acids, 1 ,
2o amines, amides) are more soluble in polar
solvents
• Nonpolar compounds (hydrocarbons, ketones,
aldehydes, ethers) tend to be more soluble in
nonpolar solvents
Bottom line
The compound to be purified should be very
soluble in the hot solvent but have very
limited solubility in the cold solvent
What makes a good
recrystallizing solvent?
A. A solvent that will NOT dissolve the solid while
at room temperature
B. The same solvent SHOULD dissolve the solid
while at elevated temperatures
C. The cold solvent must keep impurities
dissolved in it
D. Usually a good solvent can only be found by
trial and error
Choice of Recrystallization Solvents
• Water, ethanol, hexane, 2-butanone
• Place 50 mg of solid (tip of a spatula) in a test tube, add 0.5
mL (10 drops) of cold solvent
• Test as in the previous slide- solid should NOT dissolve
• The impurities should either be soluble in the cold solvent or
very insoluble in the hot solvent
• Once you find one solvent that works, you can stop looking
• Now it’s time to recrystallize about 1g
Other Solvent Characteristics
• Boiling point of the recrystallization solvent
should be ~ 50 – 120 oC
• Solvents with b.p. < 50 oC are too volatile and evaporate quickly at room temperature (e.g., diethyl ether, methylene chloride) • Solvents with b.p > 120 oC are difficult to
remove from recrystallized product
(e.g., chlorobenzene, xylenes)
The solvent should not react chemically with
• the product to be purified
– (e.g., acid chloride/ethanol, acid/amines) are
poor combinations
•
The boiling point of the solvent should be lower
than the m.p. of the product
(e.g., 1,4-dichlorobenzene (m.p 54-56 oC)/
toluene (b.p. 110 oC)
2. Dissolving the Sample
•
Always use an Erlenmeyer flask, not a beaker to carry out a
recrystallization. (hot solvent in a beaker evaporates too quickly)
Use 1 g of unknown and 50mL Erlenmeyer flask
Add a few mL of cold solvent to a flask containing the crude solid.
Warm the flask on the hot plate, using a boiling stick (in the flask)
•
Allow time for the solid to dissolve in hot solvent
•
•
Add more solvent, in small portions, while warming, trying to get all the
solid to dissolve. If after repeating this step, all the solid doesn’t
dissolve but some more of it does, repeat the process.
If all solid goes into solution, remove the flask from heat, cool to RT
(solid should reappear), place the flask in an ice-bath for 10 minutes.
– a suction filtration of the crystals, being careful what to “wet” the
Do
filter paper with.
If solid does not completely dissolve go to “Hot FIltration”
Too much solvent and your crystals don’t
come back
One key word in recrystallization is
MINIMUM Solvent Volume
Wood stick
3. Hot Filtration
•
Using a powder funnel with fluted filter paper and a flask with a small amount of
the solvent warming on the hot plate, pour the hot solution through the fluted
filter paper. Remove the flask from the hot plate. Allow to cool to room
temperature, ice and suction filter. Reminder: you don’t need to do a hot filtration
of all the solid goes into solution. In that case, go on to step 4.
4. Cooling Down the Hot Solution
• The hot-filtered solution is allowed to cool to slowly to
room temperature, then placed in cold bath (ice-water)
• Cooling the solution too rapidly will result in formation
of small crystals, which tend to be less pure compared
to crystals formed from slow crystallization
The other key word in recrystallization is:
PATIENCE!!!
Here is one problem that occurs all to often: you used too
much solvent, nothing comes out of solution upon
cooling. You put something in there, it’s still there!!!
Boil off half the solvent, and cool down again.
The hose goes to the
aspirator.
IMPORTANT– ALWAYS
REMOVE THE HOSE
FROM THE ASPIRATOR
BEFORE TURNING OFF
THE FAUCET
5. Cold Filtration
Always “wet” the paper first with the solvent
you are working with, then turn on the
aspirator.
Swirl and pour in
one motion
6. Washing & Drying the Solid
If you didn’t get all the solid out of the flask,
add more COLD solvent, scratch the solid
(if necessary) into the solvent and repeat as
above.
Once the solid is recovered, store in
student drawer to air-dry then take mp
next day or next lab period.
What did we just learn?
• Test a solvent (water or ethanol) , about
10 drops, with a small amount of your
compound in a disposable test tube. If it
dissolves, that’s not the solvent.
• If it doesn’t dissolve at RT, warm it (boiling
stick). If it doesn’t go into solution upon
boiling, that’s not your solvent.
• If it does into solution, allow to cool to RT
to allow crystallization to occur.
Continued
• If it comes out of solution, that’s the
solvent you want to crystallize all of your
acid, phenol, or neutral from.
• If it doesn’t come out of solution, and you
used ethanol as your solvent, then
DROPWISE add water until the solution
becomes cloudy and then allow to cool.
• Store all recrystallized samples without a
cap on (parafilm) to air-dry.
Expt 6 Recrystallization- A Purification Technique
Objective:
In this experiment, you will simulate a recrystallization of 4-nitroaniline, an organic solid. Data will be provided so
that you can calculate quantities of solvent needed for your experiment and to calculate a percent recovery.
Before you begin:
• Read Expt 5.0-Recrystallization in the Laboratory Manual for Organic Chemistry Chem 231/241L
(Yes the Lab Manual experiment is numbered Experiment #5 even though this is our Spring 2021 Expt 6- sorry for the
conflicting numbering systems)
• Watch both YouTube videos on recrystallization and vacuum filtration
https://www.youtube.com/watch?v=genmtAjsDzA (recrystallization)
https://www.youtube.com/watch?v=AiYnnpVUbyw (vacuum Filtration)
Structures for impure solid and potential recrystallizing solvents.
Possible
Solid to be
Structure of
recrystallizing
Recrystallized
solid
solvents
Structure of
solvent
cyclohexane
4-nitroaniline
toluene
ethanol
water
CH3CH2-OH
H2O
Solvent screening: Notes and observations from lab notebook to determine best solvent choice.
cold = room temp
hot = beaker with boiling water (100ºC)
cyclohexane
toluene
ethanol
water
cold
hot
cold
hot
cold
hot
cold
did not
dissolve
with
vigorous
shaking
not
soluble
after
several
minutes of
heating
Not good
did not
dissolve
with
vigorous
shaking
Some solid
dissolved
at boiling
after
several
minutes
Possible!
dissolved
quickly
with
little
shaking
N/A
will not work
not soluble
with much
shaking and
stirring
hot
Solid
dissolves at
boiling after
45 seconds
best choice
Data from CRC
Water solubility:
0.08 g/100 mL @ 20ºC
2.2 g / 100mL @ 100ºC
mp 146.0ºC
Instructions for your Report
1. Use solubility data provided to calculate the amount of solvent needed to purify your material. Refer to Laboratory
Manual for Organic Chemistry Chem 231/241L for example calculations.
2. After vacuum filtration and drying is complete, calculate the percent recovery based on data supplied below.
3. Provide answers to post-lab questions. (Ignore Experiment 5 Report from Lab Manual)
Report- Submit this one page only to Canvas
Calculations
1. Show your calculation to determine how much solvent is needed for recrystallization.
(5pts) a. Calculate how much solvent is needed to dissolve a 1.5 gram sample of 4-nitroaniline
(5pts) b. Calculate how much of your product will be lost to the filtrate. Assume that you used 6 mL (2 washes of 3 mL
each) of ice cold water to rinse the crystals on the filter paper.
Results
2. After drying, the mass of purified solid was 1.205 g. The mp was measured to be 143.5-145.0ºC .
(5pts) a. Calculate % recovery.
(5pts) b. Based on the mp, is your product considered to be pure?
3. Questions
(10 pts) a. Why was ethanol rejected as a recrystallization solvent?
(10 pts) b. Why was toluene rejected as a recrystallization solvent?
(10 pts) c. After most of the solid has recrystallized, why place the flask into an ice bath?
(10 pts) d. What happens if your flask is placed into an ice bath immediately after the solid has dissolved? In other
words, why not save time and skip the slow cooling of your hot solution.
Experiment 5
Recrystallization of an Organic Solid
Many solids can be purified by recrystallization, an operation in which an impure
substance is dissolved in a suitable solvent at its boiling point and then waiting until the
solution cools back to room temperature at which point the substance re-forms into a
solid and precipitates from solution. The solid that precipitates from solution usually
forms regular crystals depending on the morphology of the solid phase. Ideally, the
impurities in the original sample remain dissolved once the compound precipitates from
solution. Causing precipitation of a pure compound while the impurities remain in
solution is a physical separation. After recrystallization occurs, the pure solid is recovered
by filtration or, less effectively, a careful decantation. A slight washing of the pure
compound on the filter paper with cold solvent is done to sweep away any small amount
of impurity stuck to the pure solid. Finally, the solid is dried to constant mass and its
purity is verified by taking a melting point. Residual solvent is an impurity and will lower
the mp. Drying to constant mass is required to confirm identity.
The physical property that governs a recrystallization is solubility. Recrystallization is
based on the fact that the solubility of a solid generally increases with the temperature of
the solvent (see graph for benzoic acid solubility). However if too much solvent is used to
dissolve the impure substance, it will not be possible to recover the compound; it will
stay in solution even after the mixture has been cooled. If the solvent is boiled away or
evaporated, the solid can recovered but all the impurities will still be present; no
purification occurs.
1
For the inexperienced student, the most
common mistake in procedure is to use too
much solvent for a recrystallization. If the
solubility of a compound is known, the
amount of solvent to use for
recrystallization can be determined.
Following is an example to demonstrate
how solubility is used to determine the
volume of solvent needed to do a
recrystallization.
Consider a 3.00g sample of benzoic acid
that is contaminated with 100 mg of
acetanilide. Water is a suitable solvent to
recrystallize benzoic acid. The solubilities
for many organic compounds are readily available and often the solubility at cold and hot
temperatures is given; the data may look like this:
First we calculate how much solvent is
needed to dissolve 3.00g of benzoic acid
at the boiling point:
All of the impurity will dissolve in this
volume since 0.1 g of acetanilide (100 mg)
only require about 2 mL to dissolve:
After ingestion (all solids are dissolved), the
solution is allowed to cool. We use the low
temperature solubility value to determine
how much benzoic acid will remain dissolved:
The maximum amount we can expect to get back from our recrystallization is:
Our % recovery under ideal conditions can be calculated:
2
Using the calculation from above, 44 mL is the calculated amount for a 3 gram sample,
but in practice the actual amount of solvent may be slightly less or a little more. As long
as the final amount of solvent is not much greater than 44 mL, the sample should
eventually dissolve.
After the solid precipitates from solution, if we put our recrystalliation in an ice bath and
further lower the solubility, we could possibly squeeze out a little more product which is
the reason why a recrystallization should always be put on ice after the contents have
cooled to room temperature. Another way to increase % recovery is to take the filtrate
and reduce its volume and, try to coax more solid to precipitate; this is called a second
crop. It is not unheard of to recover a second, third and even a fourth crop when the
compound of interest is highly valuable. Typically, purity declines with the second and
subsequent crops.
If the impure compound is difficult to dissolve, any solvent that boils away needs to be
replaced until the solid goes into solution. The result may be ending up with more solvent
than needed. If the calculated amount of solvent is exceeded by a significant amount,
more solid will remain in solution which reduces % recovery. Using too much solvent at
the beginning or adding excess solvent during dissolution is the most common error.
Planning a Recrystallization
Purifying one gram or more of a solid by recrystallization can be done using a 250-mL
beaker. Using a beaker makes it easy to stir and to add more solvent while boiling the
solvent. If the solid does not readily dissolve then using an Erlenmeyer flask will lessen
the amount of solvent that evaporates. Smaller quantities of (0.1 to 1 g) can be processed
using smaller Erlenmeyer flasks or test tubes.
Dissolving the Solid
Recrystallization requires that all the solid is completely dissolved. If the hot mixture,
even at the boiling point, contains any solids or if the mixture looks cloudy, it is not
completely dissolved. You may find that adding a few drops of solvent is needed even if
you have already used the calculated amount. Be careful that you add a few drops at a
time then boil for a few minutes before adding more.
Ideally, the solid dissolves in boiling solvent and the solution is allowed to cool to about
room temperature before placing the beaker in an ice bath. Sometimes impurities will not
dissolve even at the boiling point. In these cases, a hot filtration must be done. In a hot
filtration, the boiling solution that contains your solid of interest is quickly filtered to
separate out the undissolved material. A hot filtration can be problematic if your solid
starts to crystallize on the filter paper during filtration of the boiling solution. The trick is
to warm up the glass funnel and filter flask to keep your solid in solution until the
undissolved material is caught in the filter paper.
3
Cooling the Solution
The size of crystals often depends on the rate of cooling. Rapid cooling causes many
nucleation sites which form small granular crystals. Small crystals have more surface
area to size and more impurities can adsorb to their surface. Slow cooling will allow
fewer nucleation sites to develop resulting in larger crystals. Sometimes larger crystals
can trap impurities but they are easier to clean with the wash solvent. The flask should
not be put into ice until a substantial amount of crystals have formed.
Sometimes crystallization is very slow or the solution cools but remains supersaturated. If
no crystals form, there are some techniques that can be tried.
• Use a glass stir rod to rub the inside of the beaker; sometmes the vibration will
induce crystallization, especially if you can make the glass surface squeak.
• If a sample of the pure compound is available, a few small seed crystals can be
added to provide nucleation sites.
• Dip a clean stir rod into the solution and remove. Once the solvent evaporates
from the rod a small amount of solid is left behind which is inserted back into
solution. This may be enough to seed the mixture and induce crystallization to
commence.
If no crystals form you may have used too much solvent. Concentrate the solution by
boiling off some of the solvent. Allow the solution to cool until crystallization begins.
You can also concentrate the solution under vacuum until the solution becomes cloudy.
Bring the mixture back to boiling until the cloudiness disappears. You may have to add a
few drops of clean solvent to get the cloudiness to disappear. Make sure the cloudiness is
gone then set aside and see if crystallization begins.
4
Recovering and Washing the Recrystallized Solid
The crystals are poured into a Buchner funnel
and washed with small portions of solvent to
carry away any impurities that may be
adsorbed on the solid. For small amount,
usually 1g or less, a Hirsch funnel is used with
a sidearm test tube. The filter paper is wetted
with cold solvent and momentarily hooked up
to vacuum to seal the paper on the bottome of
the ceramic funnel. If a low-boiling solvent is
used, prolonged vacuum at this stage may
evaporate the solvent and dry out the filter
paper which pulls away from the ceramic
surface.
The method that usually works best is to swirl
the beaker and pour the contents quickly into
the funnel without spilling. Vacuum should
not be attached to the sidearm at this point.
The solids will spread out and settle into a
uniform layer called the wet cake. Then gently
hold the vacuum hose on the sidearm and pull
solvent through the wet cake. As soon as wash
solvent is drawn below the top surface of the wet cake, release the vacuum from sidearm
and carefully add a small portion of cold wash solvent on top of the wet cake without
disturbing the top surface. Use approximately 3-5 mL per washing. Each time a wash is
added, vacuum should be disconnected so that a layer of fresh solvent covers entire
surface of wetcake; then apply gentle vacuum to pull the wash through the wet cake.
Usually 2-3 washings is sufficient. The vacuum hose is never attached to the ribbed
sidearm until washings are done. After the final wash has been pulled through the
wetcake vacuum is attached to the sidearm and solid is dried by airflow through the cake.
Solid may require additional drying in an oven.
Recrystalliztion from Mixed Solvent Systems
A solid that does not dissolve well in any solvent may need to be recrystallized from a
binary mixture of two solvent. There are instances where even a ternary mixture
(3-component mixture) is used for certain compounds but it is unlikely you will
encounter such examples; we will discuss 2-component solvent scenarios.
Both solvents must be miscible and the solid must be very soluble in one solvent and
mostly insoluble in the other solvent. Remember that solubility is the physical property
that governs recrystallizations. This causes us to think about polar versus nonpolar solids
and the like dissolve likes rule of thumb. There are two approaches to a binary solvent
system:
1. Both solvents are premixed in some proportion, say 50% methanol and 50%
water, and recrystallization is done as describe above.
5
2. The solid is dissolved in one solvent (#1) and the second solvent (#2) is added
dropwise to induce crystallization.
For the latter approach, the solid is dissolved in the solvent in which it is very soluble
(#1). The second solvent, in which the solid is sparingly soluble (#2), is added dropwise
until a cloudiness persists. All this is done at the boiling point. Cloudiness signals that the
solution is saturated and crystallization is imminent. In fact, cloudiness is due to a small
amount of solid that has already crystallized as tiny crystals that have yet to coalesce and
precipitate. At this point, the solvent #1 is added dropwise until the cloudiness
disappears. Then the solution is set aside and allowed to cool as before.
Choosing a Solvent for Recrystallization
Many lab procedures include the solvent to be used for the purification step. This is not
always the case. When a procedure simply states, “recrystallize the crude product”, then
the student must find one that is suitable. A suitable solvent will meet, as best as possible,
these general criteria:
• Boiling point should be in the range of 50-100°C
• Freezing point should be below -10°C
• Solvent must not bre reactive towards the solid
• Solvent is not overly hazardous or toxic to handle
• Workable solubities are 3-20g of solid per 100 mL of solvent at the bp and, less
than 2g of solid per 100 mL at cold temperatures.
Solvents listed in references sources generarlly use the terms insoluble, sparingly soluble,
soluble or very soluble, with respect to solids. If no solubility data is available then a trial
and error approach is used to find the best choice. Consider the appropriate polarity
matches between solid and solvent (like dissolves likes). Some common solvents are
listed here to begin your search. Generally, the higher the bp, the more difficult it is to
remove in the drying step.
Solvent
bp (°C)
Fp (°C)
Polarity comments
Good for polar compds; drying is slow
Water
100
0
Ethanol (95%)
78
-115
Methanol
64
-94
Good polar choice; easier to remove than
water
Good polar choice; easy to remove
Ethyl acetate
77
-84
Good for polar compds; easy to remove
Toluene
110
-95
Pet ether (mixture
of hexane isomers)
65-85
-90
Methylene chloride
103
-97
2-butanone
80
-86
Good for aromatic compds; difficult to
remove
Good for nonpolar compds and as a
co-solvent for binary solvent systems; easy
to remove
Good for moderatly polar compounds; easy
to remove; handle with care due to toxicity
Good for polar compds; higher bp than
6
acetone; relatively easy to remove
Testing Recrystallizing Solvents
Use about 100 mg of solid in a test tube and add 1 mL of solvent; stir with a
microspatula. If it dissolves, the solvent is unsuitable. If it does not dissolve, heat to
boiling with stirring and see if it dissolves; if not, then add up to 3 mL of solvent. If solid
does not dissolve in 3 mL or less, it is unsuitable.
If solid does dissolve in 1-3 mL at the boiling point, allow solution to cool and see if
crystallization occurs spontaneously or, if crystallization can be induced. If so, then you
may have a good solvent. Be sure to record the volume of solvent needed to dissolve your
solid at the bp. This is your solubility value to be used to calculate volumes for large
scale.
If no solvent is found, then consider a binary solvent system. Start with a solvent in
which the solid has a high solubility and a co-solvent in which solubility is low. Repeat
the process on small scale using 100mg in a test tube until you find a solvent system that
looks promising. Be sure to record volume of solvernt at the bp for your solubility
calculations.
Procedure
Complete solubility calculations to determine the amount of solvent needed recrystallize
about 2 grams of benzoic acid. Weigh about 2 grams of impure benzoic acid and transfer
it to a 125-ml Erlenmeyer flask. Using a graduated cylinder, add about 80-90% of
calculated amount of distilled water and bring the mixture to the boiling point by heating
on a hot plate, while stirring the mixture and boiling gently to dissolve benzoic acid
completely.
Remove the flask from the hot plate and examine the solution. If there are particles of
benzoic acid still undissolved, then add an additional amount of hot water in small
increments and resume boiling. The objective is to dissolve the entire solid in minimum
amount of solvent. Do not add too much water or the solution will not be saturated and
the yield of purified benzoic acid will be reduced. Keep adding water in small amounts
(several drops at a time from a Pasteur pipette) until all of the benzoic acid is dissolved
and the solution is boiling. Transferring boiling solvent with a pipet is not easy. Be
patient so that excess solvent is not used.
Once the solid has completely dissolved allow the flask to cool slowly. This will give the
best-shaped crystals after about 5-10 minutes. It is a good idea to cover the beaker with a
watch glass to keep out dust and debris.( Para film can be used if the beaker will be set
aside overnight in a fume hood or lab drawer.) If crystallization does not occur after 10
minutes, scrape the sides of the flask with a glass rod hard
enough to audibly scratch the interior surface of the flask. This
may dislodge small crystals that will drop into the solution and
“seed” the solution, helping to induce crystallization.
Sometimes the vibration of squeaking glass induces a
seed crystal to form. A seed crystal can serve as a
7
nucleation point for the crystallization process. After several minutes, most of your solid
should precipitate. Cool the mixture in an ice bath to maximize recovery of purified solid.
Collect your solid by vacuum filtration using a Buchner funnel with correctly-fitted filter
paper. Pour the chilled mixture into the Buchner funnel. Try to get the solid to form a
layer on top of the filter paper- this is called a wet cake. Once the solid is layered on the
filter paper, covered with another layer of solvent, then attach vacuum tube to begin
suction. The water should filter quickly. Get all the solid out of the flask using a spatula
or stirring rod. Rinsing with 1 or 2 mL of cold water helps get the crystals out of the
flask, and rinsing the wet cake helps remove impurities. Disconnect vacuum tube when
adding rinses so that rinse solvent forms a layer on top of wet cake.
Let the aspirator run for a few minutes to start air-drying the crystals. Use a spatula to lift
the filter paper and crystals out of the Buchner funnel, press them as dry as possible on a
large clean paper towel (hand dry), and allow them to dry completely. Transfer the dry
sample to a pre-weigh weighing paper or better yet, a tared container. Determine the
weigh the DRY crystals of recovered benzoic acid.
Calculate the percent recovery and obtain a mp of your purified solid.
8
Experiment 5 Report
NAME _____________________________________________
Turn in this sheet as your cover page
Include a one-page summary with:
• Identity and mass of impure solid
• Identity of recrystallizing solvent
• Calculation to determine amount of solvent needed
• Total volume of recrystallizing solvent actually used for dissolution
• Total volume of wash solvent used- record individual wash volumes in notebook
• Volume of filtrate- should be approximately equal to amount needed to dissolve
plus washes
• Mass of dry, purified solid
• Mp of pure substance
• Calculation of loss to filtrate using solubility of benzoic acid
• Percent recovery of pure compound
Include your lab notebook pages which should have entries for all the items above plus
any observations.
9
Question 5
5. Which of the following structures is ethanol?
А
B
nech
H2C
CH3
H-0-CH2CH3
H3C
0–CH2CH3
ОА
o
C
Question 1
1 pts
1. What are the four types of hydrocarbons?
A. Alkanes, alcohols, alkenes, halogens
B. Alkanes, alkenes, alcohols, aromatic compounds
C. Single bonds, double bonds, triple bonds, cyclic compounds
D. Alkanes, alkenes, alkynes, aromatic compounds
E. Alkanes, alkenes, alkynes, alcohols
A
A
B
E
Question 2
1 pts
2. Most hydrocarbons will dissolve in polar solvents such as water and alcohols.
T
F
True
False
>
Question 3
1 pts
3. What is the general formula for an alkane?
CnH2n+2
CnH2n
CnH2n-2
O CnHn
Question 4
1 pts
4. What is the molecular formula for an alkane with 7 carbon atoms?
C6H14
C7H14
C7H16
C7H28
Question 5
1 pts
5. Determine the molecular formula for the following molecule:
C6H14
C6H4
C6H8
O C4H8C2
Question 6
5 pts
6. Match each element to its normal bonding pattern
с
[Choose ]
I
[Choose]
>
N
[Choose ]
< o [Choose] >
F
[Choose ]
7. Indicate the formal charge on any structure that contains an atom that does not have its normal
bonding pattern. Be sure to select zero for any structure that does not have any charges.
E F
H3C
CHE
CHE
C-CH,CH, tô-CH,CH, CH,-c: H-NSCH CHO
ін, CHA
CHE
CH,
B
C
D
CH
CH
А
[Choose
< B [ Choose с [ Choose < D [ Choose < E ( Choose < F ( Choose G [ Choose < 8. Examine this image then answer each question: C. What is this C-CC bond angle? B. What is the hybridization of this carbon atom? CI 14 CH I NH A. What kind of alcohol group is this 1° 2° 3° ? HO Br D. Is this double bond cis or trans? E. What kind of alkyl bromide is this 1° 2° 3' ? F. Although none are drawn, how many lone pair electrons are in this structure? A [Choose] B Choose с Choose < D [Choose] E Choose] V F [ Choose >
Question 9
10 pts
9. Identify the functional group in each molecule.
CH
CH-CH2
moet worden
Гн
6-O
CH.CH
CH
A
B
C
OH
OH
CH.CH
CH
сн.
N
CH,CH
CH-CEC-
CH,
CH,
ז
1
А
Choose
Choose
с
Choose
D
Choose
E
Choose
>
F
Choose
V
C
Choose
Choose
V
Choose
>
Choose
< Question 10 1 pts 10. Which Newman projection is the eclipsed conformation, A or B? CH CH3 H H CH3 ਨਾ CH3 Н. H HH H A B ОА O 2 Question 11 3 pts 11. Identify each conformation: CH, CH3 CHY Сн, H H H CH3 H H H HH H H Н. CH3 А. B с A [Choose ] B [Choose ] с [Choose ] Question 10 1 pts 10. Which Newman projection is the eclipsed conformation, A or B? CH CH3 H H CH3 ਨਾ CH3 Н. H HH H A B ОА O Question 10 1 pts 10. Which Newman projection is the eclipsed conformation, A or B? CH CH3 H H CH3 ਨਾ CH3 Н. H HH H A B ОА O 2 Question 11 3 pts 11. Identify each conformation: CH, CH3 CHY Сн, H H H CH3 H H H HH H H Н. CH3 А. B с A [Choose ] B [Choose ] с [Choose ] Question 12 1 pts 12. Label each structure (A, B, C) as cis or trans. CH3 CH3 CH3 H pin Br H H Br CH3 А B с A(trans), B (trans), C (trans) O A (cis), B (cis), C (trans) O A(trans), B (cis), C (trans) O A(trans), B (cis), C (cis) None: these are not cis-trans isomers Question 13 3 pts 13. Use scratch paper if needed. You may need to draw the ring flip for both isomers to verify your answer. H H Which stereoisomer has lower energy, cis- or trans-1,2- dimethylcyclohexane? H2C CH3 CH3 H CH3 the cis isomer the trans isomer they are equal in energy 14. Cyclohexane stereochemistry Indicate the position (ax or eq) for the following groups: A B с D E E B A [Choose ] B [Choose] с [Choose ] [Choose] E [Choose ] 15. More cyclohexane stereochemistry Indicate the relationship (cis or trans) for the following groups: D A and B A and C B and C E C and E B B and D A and B [Choose ] A and C [Choose ] V B and C [Choose ] C and E [Choose ] B and D [Choose ] Question 16 3 pts 16. Which of the following structures show the correct stereochemistry after the ring- flip? Н CH, Br CI CH3 Br H B H Br Br CH3 CHE Br H CI CH3 D O 0 0 Question 17 1 pts 17. Which of the following molecules are chiral? CH CI CHE Br CH H2C un VOH NH2 H OH A B с D E All of them are chiral O A, C, and E are chiral O A, C, D and E are chiral B, C and E are chiral Only A and B are chiral Non of these are chiral, they are enantiomers >
Question 18
3 pts
18. Assign absolute configuration to each molecule:
CH3
H
CN
CH3
Н.
salil
Br
A
B
он
с
A
[Choose ]
В
[Choose ]
< с [Choose ] Question 16 3 pts 16. Which of the following structures show the correct stereochemistry after the ring- flip? Н CH, Br CI CH3 Br H B H Br Br CH3 CHE Br H CI CH3 D O 0 Question 19 2 pts Br, 19. Consider cis-1,2-dichlorocyclopentane and trans-1,2-dichlorocyclopentane: Br Br Br A. Which isomer can be described as meso- cis or trans or both or neither? cis trans B. Which isomer exists as a pair of enantiomers- cis or trans or both or neither? A [Choose ] B [Choose ]