1. No category

CHEM1902/4 Worksheet 7 – Answers to Critical Thinking Questions
The worksheets are available in the tutorials and form an integral part of the learning outcomes and
experience for this unit.
Model 12: Addition to a Carbonyl Group
δ−
O
δ+
N
O
H+
N
1.
See below.
O
(i) LiAlH4
OH
N
OH
+
(ii) H /H2O
A
OH
O
+
(i) CH3CH2MgBr
(ii) H /H2O
B
2.
The first product is chiral and a racemic mixture will be produced. The second product is achiral.
3.
Intermediates C and D have a leaving group attached to the tetrahedral carbon (the carbon-that-wasthe-C=O), unlike A and B above. So C and D are likely react as shown below, forming E and F.
O
O
(i) LiAlH4
H
H
Cl
C
O
O
(i) CH3CH2MgBr
OCH3
D
4.
E is an aldehyde, F is a ketone. These are likely to react with the nucleophilic reagents present
(LiAlH4 and CH3CH2MgBr), so E would be reduced to the primary alcohol [CH3CH2CH2OH], F
converted to the tertiary alcohol [(CH3CH2)3COH].
Aldehydes are more electrophilic than the corresponding esters. There is resonance stabilization of
the ester (see below), which acts to reduce the partial positive charge on the carbonyl C atom.
O
O
OCH3
OCH3
5.
–
++ –
R–Mg–Br
6. See below.
MgBr
(i)
HO
O
(i)
MgBr
OH
+
(ii) H /H2O
+
(ii) H /H2O
7.
The acid converts the metal salt (ROMgBr and RCOOMgBr respectively) to the products shown.
Grignard reagents act like R- MgBr+: a carbanion is a very strong base and will react vigourously
with any source of H+ to produce RH and heat.
8.
See below
O
O
CH4 +
CH4 +
O
+ MgBr2
+ MgBr2
Model 13: Carboxylic Acid Derivatives
1. See below.
H+
O
O
H+
H
O
OH
OH
H
OH
O
H
-H+
HO
H
O
-H+
O
O
H
-H2O
O
H
OH
O
O
The Lewis structure of thionyl chloride, SOCl2, is shown below. The S has a significant δ+ charge. It reacts
with a nucleophile like water via nucleophilic attack of H2O on the S atom:
Ultimately, this leads to SO2 and HCl. The first step of the reaction with carboxylic acids is similar:
Ultimately, this leads to SO2 and RCOCl.
2.
See below. The base, B, could be solvent or H2NCH3.
O
O
O
- Cl-
Cl
Cl
NH2CH3
NHCH3
H
H2NCH3
B
O
NHCH3
3.
The reactivity increases in the order:
amides < acids ~ esters < anhydrides < acid chlorides
The resonance stabilization shown in Model 12 Q4 is key. As nitrogen is less electronegative
than oxygen, it is more able to donate it lone pair and so the second resonance form is more
important in amides than in acid anhydrides and esters.
The resonance stabilization requires good overlap of the p-orbital holding the lone pair and the
C=O π orbital.
As Cl is larger than O or N, the C-Cl bond is long and the overlap of these orbitals is poor: little
resonance stabilization occurs in the acid chlorides. The electron-withdrawing nature of the
electronegative halide dominates increasing the δ+ charge on the carbonyl carbon atom.
4.
Brønsted acid / base H+ transfer reactions occur:
O
KOH
O
+ H2O
O
H2
N
amine
O
+
Extension
5.
An amine will react with a carboxylic acid in a simple acid-base reaction as shown in Q3. The
amine will attack the most δ+ atom in the carboxylic acid, which is the –OH proton rather than
the carbonyl carbon atom.
See below.
H
O
R
OH2
OH
OH
OR'
OR'
OR' H
OH2
OH
H
H2O
OH2
O
H3O
OH
H
H2O
O
OR'
R
OH
R
OH
OH
H
+ R'OH
6.
The steps are the reverse of those in the acid catalysed formation of an ester. As shown above,
each step is reversible and it is possible to drive the reaction towards carboxylic acid + alcohol
or towards ester by changing the conditions.
Extension
See below.
O
O
O
OR'
OR'
R
O
H
OH
+ R'O
OH
proton
transf er
O
+ R'OH
R
O