Download CHEM120R STEREOCHEMISTRY

CHEM 120R / 2012
Organic Chemistry Tutorial 2 - Solutions
School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus
Chemical Reactivity - CHEM120R
STEREOCHEMISTRY - SOLUTIONS
1.
Draw out and name the 6 isomeric pentenes in both full structural formula and in line-bond
structures.
1-pentene
2.
E-2-pentene
Z-2-pentene
2-methyl-1-butene
3-methyl-1-butene
2-methyl-2-butene
Draw out & name the cis-(Z) and trans-(E) isomers of 5-chloro-2-pentene.
Cl
Cl
E-5-chloro-2-pentene
3.
Z-5-chloro-2-pentene
Which of the following compounds has a pair of cis-trans isomers? Give reasons for your
answer, and deduce the general rule involved.
H
Br
Br
H
H
Br
H
Br
CHBr=CHBr - there must be 2 different groups attached to each carbon of the double bond.
4.
5.
Name the following alkenes, including the E-Z configuration.
a)
Z-pent-2-ene
b)
E-4-bromo-1-chloropent-1-ene
c)
E-3-methylpent-2-en-2-ol
d)
Z-2-ethyl-1,3-dimethylcyclohexene
e)
E-1-chlorocyclohexene
f)
3-ethyl-4-methylhex-3-en-2-one
How many asymmetrical carbons are there in menthol?
THREE
OH
H3C
CH(CH3)2
Page 1 of 5
CHEM 120R / 2012
6.
Organic Chemistry Tutorial 2 - Solutions
Determine the number of asymmetrical centres (chiral carbons) in the following molecules
and predict the number of possible stereoisomers.
a)
CH2(OH)CH(OH)CH(OH)CHO - 4 isomers, namely R,R; R,S; S,R and S,S.
HOH2C
H
OH
R
HOH2C
R
R
HO
H
H
OH
b)
7.
HOH2C
S
H
HO
C H
O
OH
H
S
R
HO
H
C H
O
OH
H
S
S
H
HO
C H
HOH2C
C H
O
O
HO2C-CH(Cl)-CH(OH)-CO2H - 4 - same as above.
Using the Cahn-Ingold-Prelog, assign absolute (R or S) configurations to each of the following
structures, and then name them:
a)
CH2CH3
H C
Br
8.
b)
H2N
H3C
CH3
c)
H
C
CO2H
C
H3C
Br
C
H
CH2CH3
CH3
H
a)
R-2-bromobutane
b)
S-2-aminopropanoic acid
c)
R-2-chlorobutane
d)
S-1-bromo-1-cyclopropylethane
Draw structures to represent the following molecules:
a)
R-pentan-2-ol
b)
S-3-amino-2,3-dimethylhexanoic acid
CH2CH2CH3
OH
H
CH3
c)
CH3
H2N
CH2CH2CH3
S-2-chloropropanal
H
d)
CH(CH3)CO2H
R,R-butan-2,3-diol
OH
C O
H
CH3
9.
d)
Cl
OH
H
CH3
Cl
H
CH3
Draw Fischer projections of the following molecules:
a)
CH2CH3
H C CH
3
Br
b)
CH2CH3
H3C
c)
C H
Br
d)
H
H2N C CO H
2
H3C
H
H C
C OH
CH3
O
CH2CH3
Br
H
CH3
CH2CH3
H
Br
CH3
H
CH3
H
NH2
CO2H
Page 2 of 5
HO
CH3
CHO
CHEM 120R / 2012
10.
Organic Chemistry Tutorial 2 - Solutions
Draw Newman projections for the staggered and eclipsed conformations of:
a)
1,2-dibromoethane
(C1-C2 axis)
C1
H
H
Br
Br
H
CH2CH3
H
C2
b)
3-methylpentanoic acid (C2-C3 axis)
O
CH3
OH
C3
11.
H
C2
CO2H
H
H
CH2CH3
In the structures in question 5 of the previous organic tutorial, identify as primary (1o);
secondary (2o); tertiary (3o) or quaternary (4o):
12.
a)
the alcohols in – 5(j) - 2o; 5(n) - 1o; and 5(o) - 2o;
b)
the amines in – 5(g) - 1o; 5(h) - 3o; 5(j) - 1o; and 5(t) is an amide - 3o; and
c)
the alkyl halides in – 5(l) - 2o, and 5(u) - 2o.
Draw, and name, ALL of the plausible Lewis structures for the isomers of:
a)
C5H10O
(including E & Z forms)
i)
Cyclic alcohols - 1 structure
OH
ii)
Alkenols - 23 structures including E & Z forms
HO
HO
HO
pent-4-en-1-ol
E-pent-3-en-1-ol
Z-pent-3-en-1-ol
OH
pent-4-en-2-ol
Z-pent-3-en-1-ol
OH
E-pent-3-en-2-ol
E-pent-2-en-1-ol
HO
HO
HO
HO
E-pent-1-en-1-ol
Z-pent-1-en-1-ol
OH
OH
Z-pent-3-en-2-ol
Z-pent-2-en-2-ol
Page 3 of 5
HO
E-pent-2-en-2-ol
OH
pent-4-en-2-ol
CHEM 120R / 2012
Organic Chemistry Tutorial 2 - Solutions
OH
OH
OH
pent-1-en-3-ol
Z-pent-2-en-3-ol
E-pent-2-en-3-ol
HO
HO
HO
HO
E-3-methylbut-1-en-1-ol
Z-3-methylbut-1-en-1-ol
OH
OH
3-methylbut-1-en-2-ol
iii)
3-methylbut-2-en-1-ol
OH
3-methylbut-2-en-2-ol
3-methylbut-3-en-2-ol
Alkene-ethers - 11 structures including E & Z isomers!
O
O
4-methoxybut-1-ene
O
E-2-methoxybut-2-ene Z-2-methoxybut-2-ene E-1-methoxybut-1-ene Z-1-methoxybut-1-ene
O
3-methoxy-2-methylprop-1-ene
O
1-methoxy-2-methylprop-1-ene
O
O
3-ethoxyprop-1-ene
E-1-ethoxyprop-1-ene
O
Z-1-ethoxyprop-1-ene
2-ethoxyprop-1-ene
Aldehydes - 5 structures
O
O
O
O
O
H
H
H
pentanal
v)
O
O
O
iv)
3-methylbut-3-en-1-ol
3-methylbutanal
H
H
H
H
R-2-methylbutanal
S-2methylbutanal
Ketones - 2 structures
O
O
pentan-2-one
3-methylbutan-2-one
Page 4 of 5
2,2-dimethylpropanal
CHEM 120R / 2012
Organic Chemistry Tutorial 2 - Solutions
Polarity, Intermolecular Forces & Boiling Points:
The C-O, C=O and O-H bonds are polar, resulting in all of the above structures being polar
molecules, but due to the differences in the bonding, the there are different strengths of
polarities. The alcohols with the O-H bonds are the most polar as they are able to hydrogen
bond which gives rise to strong intermolecular forces and as a consequence, the highest
boiling points. within this group (100-140oC), the more of a straight chain the molecule has as
opposed to shorter parent chain with branches, the higher the boiling points are.
The
aldehydes and the ketones with the polar carbonyl group are second in terms of strength of
intermolecular forces, with boiling points for the above structures ranging from 60-80 oC. The
weakest intermolecular forces occur within the ethers, and will therefore have the lowest
boiling points, which for the above structures range from 30-45 oC.
b)
C5H12O
(including enantiomers)
i)
Alcohols - 10 structures
HO
OH
OH
1-pentanol
OH
H
HO
H
R-2-pentanol
H
S-2-pentanol
3-pentanol
R-2-methylbutan-1-ol
OH
H
OH
HO
OH
S-2-methylbutan-1-ol
ii)
H
HO
3-methylbutan-1-ol
2-methylbutan-2-ol
H
R-3-methylbutan-2-ol
S-3-methylbutan-2-ol
Ethers - 6 structures
O
O
O
O
1-methoxybutane
1-methoxy-2-methylpropane
1-ethoxypropane
H
H
O
O
S-2-methoxybutane
Polarity, Intermolecular Forces & Boiling Points:
Same order as described above.
Page 5 of 5
R-2-methoxybutane
2-ethoxypropane