Transcript Organic Chemistry HL

Organic Chemistry
HL only
20.1 Introduction
More functional groups
Esters
O
Contain functional group
COOR, where R = an alkyl
group such as CH3 (methyl
group).
C
OR
Naming esters
O
name of this alkyl
group forms first part
of name.
R C
O R
second part of name is derived
from the name of the carboxylic
acid salt.
Name the following ester
ethyl propanoate
O
CH3CH2 C
this is an ethyl group.
O CH2CH3
this is a propanoate group.
Name the following ester
methyl methanoate
O
this is a methyl
group.
H C
O CH3
this is a methanoate group.
Draw out the structure of
ethyl benzenecarboxylate
O
C 6 H5 C
O CH2CH3
Draw out the structure of
ethyl butanoate
O
CH3CH2CH2 C
O CH2CH3
O
C
Amide
NH2
suffix –amide
For example
CH3CONH2
ethanamide
We will also come across amides where one of
the hydrogen atoms attached to the N atom is
replaced by an alkyl group these are referred
to as N-substituted amides.
The structure shown is N-methylethanamide.
O
CH3C
NHCH3
C N
Nitriles
suffix -nitrile
For example
CH3CH2CN
propanenitrile
Remember to include the nitrile group as part of
the C chain.
C NH2 Amines
suffix –amine or prefix amino-
For example
CH3NH2
methylamine
H2NCH2COOH aminoethanoic
acid
-amine suffix tends to be only used for short chain
amines e.g. propylamine.
Amino- prefix is used for longer names so
CH3CH2CH2CH2CH2CH2NH2 is 1-aminohexane
What would the structure of 2-aminohexane look
like?
For secondary and tertiary amines, the longest
alkyl chain attached to the N is identified and we
then name in a similar fashion to N-substituted
amides. So for example:
H
N-methylethanamine
N
CH3 C2H5
H
N-ethylpropanamine
CH3
N
CH3
N
C2H5 C3H7
N,N-dimethylethanamine
C2H5
20.2 Nucleophilic Substitution
Reactions
The examples of nucleophilic substitution looked
at earlier are not the only examples.
Other nucleophiles which will react with
halogenoalkanes include H2O, NH3 and CN-.
Using water as the nucleophile would produce an
alcohol but the reaction is much slower than
with hydroxide ions as the hydroxide ions have
a negative charge so are attracted more
strongly to the d+ on the C atom.
The order of reactivity with these nucleophiles is:
C – I > C – Br > C – Cl
As explained earlier, this is due to increasing bond
strengths.
From experimentation we also found:
tertiary > secondary > primary
This is harder to explain but it is thought that the
activation energy to form the tertiary carbocation
intermediate in SN1 is less than that required to
form the transition state in SN2.
1. With cyanide ions
CH3CH2I (ethanol) + CN-(aq)
CH3CH2CN + Ipropanenitrile
This is an important reaction for organic
chemists as it increases the length of the
carbon chain. The cyanide group can then
be converted to other functional groups:
CH3CH2CN
H+
H2O
H2
Ni
CH3CH2COOH
propanoic acid
CH3CH2CH2NH2
1-aminopropane or
propylamine
The mechanism for the nucleophilic substitution
reaction is:
H
CN-CH
3
NC
C
H
-
H
Br
C
Br
Transition
state
CH3 H
H
NC
C
H
SN2
CH3
+
-
Br
2. With ammonia
CH3CH2Br + NH3
CH3CH2NH2 + HBr
aminoethane
H
NH3 CH
3
C
H
Br
H
H
H2N
C
Br
Transition
state
CH3 H
H
H2N
C
H
SN2
CH3
+
-
Br
There is still a lone pair of electrons on the N
atom in ethylamine so this can react with a
further molecule of bromoethane to form a
secondary amine.
C2H5NH2 + C2H5Br  (C2H5)2NH + HBr
N-ethylethanamine
The secondary amine still contains a lone pair
so further reaction occurs forming a tertiary
amine.
(C2H5)2NH + C2H5Br  (C2H5)3N + HBr
N,N-diethylethanamine
The tertiary amine still contains a lone pair so
further reaction occurs forming a quaternary
ammonium salt.
(C2H5)3N + C2H5Br  (C2H5)4N+Brtetraethylammonium
bromide
A better method for making a primary amine
involves catalytic (Ni catalyst) hydrogenation of
a nitrile. The yield is larger and there is no
further reaction possible.
CH3CH2CN + 2H2  CH3CH2CH2NH2
propanenitrile
propylamine
20.3 Elimination Reactions
When warm aqueous sodium hydroxide reacts
with a halogenoalkane, an alcohol is formed via
a nucleophilic substitution reaction. The
hydroxide ions behave as a nucleophile so:
C2H5Br + OH-(aq)  C2H5OH + BrHowever, if the sodium hydroxide is dissolved in
hot ethanol and the mixture heated under reflux a
different product is formed, an alkene.
This is an elimination reaction and the sodium
hydroxide is behaving as a base (proton
acceptor) rather than a nucleophile.
C2H5Br + OH-(alc)  C2H4 + H2O + Br-
Overall HBr is eliminated (removed) from the
halogenoalkane.
There are two possible mechanisms known as
E1 and E2.
H
H
CH3 C
C
Br
H
H
H
H
CH3 C
+
C
E1
H
H
acting as a base
OH -
H
H
CH3 C
C
Br
H
E2
H
H
H
C
CH3
OH acting as a base
propene
C
H
Br -
H
OH
nucleophilic substitution
alcohol
RCH3CH2OH + X-
+ OH- (aqueous) hydroxide acts as a nucleophile
RCH2CH2X
+ OH- (ethanol)
elimination
hydroxide acts as a base
RCH=CH2 + H2O + Xalkene
20.4 Condensation Reactions
A condensation reaction involves two molecules
reacting together to form a larger molecule with
the elimination of a small molecule such as water
or hydrogen chloride.
Esterification is an example of a condensation
reaction. Esterification involves the formation of
an ester from a carboxylic acid and an alcohol in
the presence of concentrated sulphuric acid.
For example, ethyl ethanoate is formed when
ethanoic acid and ethanol are mixed in the
presence of a few drops of concentrated sulphuric
acid.
CH3COOH + C2H5OH Ý CH3COOC2H5 + H2O
The conc sulphuric acid acts as a catalyst and
also shifts the equilibrium to the right hand side
by removing water.
The water is formed from the –OH of the
carboxylic acid combining with the H from the
alcohol.
Uses of Esters
Solvents – esters are volatile and polar. Polarity means
that they act as solvents for many polar organic
compounds. Low b.p. means that they evaporate from
less volatile solutes. e.g. ethyl ethanoate is used as the
solvent in glues such as polystyrene cement.
Plasticisers – plastics are often not flexible as chains
cannot move over each other easily. The addition of
plasticisers allows chain movement. Over time these
additives escape so the plastic becomes brittle and stiff.
Food flavourings – many esters have a sweet, often
fruity smell and are used as artificial food flavouring.
For example –
pentyl ethanoate – pear
2,2,dimethylpropyl ethanoate – banana
octyl ethanoate – orange
ethyl butanoate – pineapple
pentyl pentanoate – apple
Amide Formation
Carboxylic acids also undergo a condensation
reaction with amines (or ammonia) to form an
amide. The –OH group from the carboxylic acid
reacts with one of the hydrogen atoms attached to
the N atom to form water.
RCOOH + R’NH2  RCONHR’ + H2O
R’ = an H atom  amide
= an alkyl group  N-substituted amide
O
CH3C
+ NH3 
OH
O
CH3C + CH3NH2 
OH
O
CH3C
+ H2O
NH2
O
CH3C
+ H2O
NHCH3
Why is this reaction of biological importance?
H
R
primary
amino group
C
carboxylic
acid group
COOH
NH2
Two amino acid molecules can react together to
form an amide.
What is the correct IUPAC name of the two
amino acids on the next slide?
Deduce the structure of two possible
condensation products.
H
H
C
COOH
glycine
COOH
alanine
NH2
H
CH3
C
NH2
The two amino acid units are held together by
what is known as the peptide bond (peptide link).
O
C N
H
The two condensation products are known as
dipeptides.
H2NCHCOOH + H2NCHCOOH
R1
R2
peptide link
O
H2NCH – C – N – CHCOOH
R1
H
R2
+ H2O
On one end of the dipeptide is an amine group
and on the other is a carboxylic acid group so
further reaction at each end is possible. The
result is a CONDENSATION POLYMER. In
this instance a polyamide.
Condensation Polymers
Condensation polymers are formed by the
reaction between molecules having two
functional groups, involving the loss of
small molecules such as H2O, CH3OH or
HCl.
There are two types of condensation polymer:
• polyamides
• polyesters
Polyamides
The reaction between a dicarboxylic acid
and a diamine leads to the formation of a
polyamide.
For example
Nylon-6,6 is formed from hexanedioic
acid and hexane-1,6-diamine.
Nylon-6,6 is so called as it is made from a
6 C diacid and a 6 C diamine.
O
O
nHOC(CH2)4COH + nH2N(CH2)6NH2
O
O
O
O
- C(CH2)4C HN(CH2)6NHC(CH2)4C HN(CH2)6NH repeating unit
n
+ 2n-1H2O
As with naturally occurring polyamides
(polypeptides and proteins), synthetic
polyamides are susceptible to hydrolysis
and can be broken down into component
monomer units. Consequently,
polyamides are biodegradable.
Polyesters
The reaction between a dicarboxylic acid
and a diol leads to the formation of a
polyester.
The most important example is terylene
which can be formed from benzene-1,4dicarboxylic acid and ethane-1,2-diol.
O
O
HOC
COH
+
HOCH2CH2OH
benzene-1,4-dicarboxylic acid
ethane-1,2-diol
-H2O
O
O
O
O
OC
C
OCH2CH2O C
C
OCH2CH2O
n
+ 2n-1H2O
Polyesters (like esters) are susceptible to
hydrolysis and can be broken down into
their component monomers. So polyesters
(like polyamides) are biodegradeable.
20.6 Stereoisomerism
Stereoisomers are compounds that have the
same structural formula but their atoms are
arranged differently in space.
There are two types:
1.Geometrical isomerism and,
2.Optical isomerism.
Geometrical Isomerism
Arises due to lack of rotation at C = C in alkenes.
H
H
R
C=C
C=C
R
cis isomer
H
R
H
R
trans isomer
It is not possible to have geometrical isomerism
when there are two identical groups attached to the
same C atom in the double bond. So but-1-ene does
not exist as geometric isomers whereas but-2-ene
does.
The C atoms in the double bond are sp2
hybridised.
One of the bonds is a  bond formed by overlap
of two sp2 hybrid orbitals.
The other is a  bond formed by sideways overlap
of p orbitals. These must be in the same plane to
overlap. Any attempt to rotate will mean that
these will no longer be in the same plane which
would mean the bond would be broken.
Consequences of Geometric Isomerism
Melting points are influenced by how closely
molecules pack together.
cis but-2-ene melts at -139 °C, trans but-2-ene
melts at -106 °C.
cis 1,2-dichloroethene melts at 60 °C, trans 1,2dichloroethene melts at 48 °C.
Draw out the cis and trans isomers of but-2-ene1,4-dioic acid.
Which of the two isomers will have the highest
melting point? Why?
trans isomer has an m.p. of 286 °C, cis isomer
has m.p. of 131 °C.
trans isomer has strong hydrogen bonding
between molecules but cis isomer has strong
hydrogen bonding within molecules.
Which of the two isomers will dehydrate more
easily? Why? What will the product be?
Geometric isomerism is also found in cyclic
compounds where the rigid structure of the ring
prevents free rotation.
For example: 1,2-dichlorocyclopropane exists as
cis and trans isomers.
Draw them!
How many isomers are there of
dichlorocyclobutane? Draw them and name them.
Optical Isomerism
Arises when there are four different groups attached to
a carbon atom.
This means that the molecule
has no centre, plane or axis of
symmetry.
The molecule is said to be
CHIRAL and possesses an
asymmetric carbon atom.
Two tetrahedral arrangements in
space are possible so that one is
the mirror image of the other.
For example 2-hydroxypropanenitrile
H3C
CH3
C
C
CN
H
NC
OH
H
HO
mirror
Stereoisomers of this type are referred to as enantiomers.
Enantiomers have exactly the same physical properties
except for their effect on the plane of plane-polarised
light. As a result they are referred to as optically active.
Plane polarised light is made of waves vibrating in one
plane only. When it is passed through a solution of a
chiral molecule, the light emerges with its direction of
polarisation changed.
One enantiomer rotates the light in a clockwise direction
and is referred to as the (+) isomer or dextrorotatory (d).
Its mirror image rotates the light by exactly the same
angle but in an anti-clockwise direction. It is referred to
as the (-) isomer or laevorotatory (l).
A mixture of equal amounts of both enantiomers is
optically inactive because the effects of each enantiomer is
cancelled out. Such a mixture is called a racemic mixture
or racemate.
Draw out the structures of the following molecules and state
whether they are optically active or not. If they are
optically active, highlight any chiral carbon atoms.
1. Butan-2-ol
4. 2-aminopropanoic acid
2. 3-methylhexane
5. 1-aminopropan-2-ol
3. 3-methylpentan-3-ol
6. 2-methylpropan-2-ol
H H OH H
H C C C
C H
butan-2-ol
H H H H
H H H CH3 H H
H C C C
C C C
H H H H H H
H
3-methylhexane
H H CH3 H H
H C C
C C C H
H H OH H H
H NH2
H C C
H H
3-methylpentan-3-ol
O
C
OH
2-aminopropanoic
acid
H OH H
H C
C C NH2
H H H
1-aminopropan-2-ol
H OH H
H C
C C H
H CH3 H
2-methylpropan-2-ol
Many naturally occurring molecules exist as single enantiomers,
for example most amino acids, such as 2-aminopropanoic acid
(alanine).
The chemical properties of enantiomers are identical except in
reactions with other optically active substances.
Enzymes are stereospecific. This means they will catalyse the
reactions of only one of a pair of isomers.
Acts much faster when
administered as the
(+)-enantiomer.
(+)-carvone is
found in spearmint
oil, (-)-carvone is
the main constituent
of caraway seed oil.
thalidomide