Download 3.Carboxylic acid derivatives

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20/08/2013
Carboxylic acid derivatives
Acyl Chlorides and Acid
Anhydrides
N Goalby
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Carboxylic acid derivatives
H
carboxylic acid
(ethanoic acid)
H
O
C
H
O
C
C
H
O
H
H
O
C
C
H
H
O
CH3
Ester
(ethyl ethanoate)
C
O
O
Acid Anhydride
(ethanoic anhydride)
C
H3C
C
Cl
O
O
Acyl Chloride
(ethanoyl chloride)
key
H
H
H
CH 3
H
C
CH3
Amide
(ethanamide)
C
NH2
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20/08/2013
Comparing the reactivity of the acid derivatives
Many of the reactions of the carboxylic acid derivatives
follow the pattern below with an attack by an nucleophile.
O
CH3
C
O
+
:X-
CH3
+ :W-
C
W
X
Where –W: and :W- can be –Cl and Cl-
(acyl chlorides)
`or
-OCH3 and OCH3-
(esters)
`or
-NH2 and NH2-
(amides)
On a simplistic level, the stronger the electron attracting
power of ‘W’, the more positive the carbon, and the more
attractive the carbon is to nucleophiles.
extra
Comparing the reactivity of the acid derivatives
The relative attractive powers of the –W: are
-Cl > -OH > -OCH3 > -NH2
Therefore in the case of hydrolysis reactions, acyl chlorides
are highly reactive and will be hydrolysed by weak
nucleophiles such as water.
Amides and Esters contain only weak electron attracting W
groups and need strong nucleophiles such as hydroxide ions
in NaOH to hydrolyse.
This difference in reactivity is caused by a combination of the
electronegativity of the Cl’s, N’s and O’s causing electron density to be
withdrawn from the carbon and delocalisation of the lone pairs on these
atoms into the carbonyl system
extra
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Comparing the reactivity of Acyl chlorides to amides
O
Acyl Chloride
CH 3
O
Amide
C
CH3
C
Cl
NH2
Acyl chloride are more reactive than amides
Cl and N have similar electronegativities and so should
attract electron density from the carbon by similar amounts,
making the carbons equally positive
However, the lone pair on the nitrogen delocalises with the
carbonyl group which decreases its reactivity
The Cl is too big to be able to delocalise. This difference in
the ability to delocalise explains the difference in reactivity
extra
Reactions of Acyl Chlorides
Acyl chlorides are much more reactive than carboxylic acids.
The Cl atom is electronegative and attracts electrons from
the carbon. The bond becomes polar . (Combined with the
inability to delocalise unlike the OH group in carboxylic
acids).
The carbon is therefore made
positive and so is more
attractive to nucleophiles than
in carboxylic acids.
So less severe conditions are
needed to bring about
nucleophilic reactions in
comparison to a carboxylic
acid.
O δCH3
δ+
C
Cl
δ-
Nu
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20/08/2013
Acid Anhydrides
Generalised structure of acid
anhydride
O
R
Ethanoic anhydride
O
CH3
C
C
O
O
R
H3C
C
C
O
O
Acid anhydrides have a similar reactivity to acyl chlorides and
therefore bring about the same changes in functional groups.
The main difference is the by-products. Acyl chlorides mostly
give off HCl. Acid anhydrides give off RCOOH
Acyl chloride :Reaction with Water
Change in functional group: acyl chloride
Reagent: water
Conditions: room temp.
H
O
CH3
carboxylic acid
+ H2O
C
Cl
RCOCl (l) + H2O
H
C
H
O
C
+ HCl (g)
O
H
RCO2H + HCl (g)
Observation: Steamy white fumes of HCl are given off
The nucleophile is the lone pair of electrons on the oxygen
in the water molecule.
Type of reaction: addition-elimination
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Acid anhydride: Reaction with Water
Change in functional group: acid anhydride
Reagent: water
Conditions: room temp.
(RCO)2O + H2O
2RCO2H
O
H
CH3 C
+ H2O
O
H3C
carboxylic acid
2
H
O
C
C
C
O
H
H
O
Nucleophilic Addition Elimination Mechanism
δO
δ+
CH3 C
Cl
nucleophilic
addition
O
CH3 C
+
OH
Cl H
OH
elimination
H
CH3
CH3
O
C
H
OH
H
O
C +
OH
Cl
Cl
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Nucleophilic Addition Elimination Mechanism
Exam mark scheme version
acyl chloride: reaction with alcohols
Change in functional group: acyl chloride
ester
Reagents: alcohol
Conditions: room temp ( less severe conditions than
esterification of a carboxylic acid where sulphuric acid
catalyst is needed and heat under reflux)
O
CH3
C
+
H
H
H
C
C
H
H
O
H
Cl
H
H
O
C
C
H
O
H
H
C
C
H
H
H
+ HCl (g)
Observation: Steamy white fumes of HCl are given off
RCOCl (l) + CH3CH2OH
RCO2CH2CH3 + HCl
The nucleophile is the lone pair of electrons on the oxygen in
the alcohol molecule.
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Acid anhydride: reaction with alcohols
Change in functional group: acid anhydride
ester
Reagents: alcohol
Conditions: room temp ( less severe conditions than
esterification of a carboxylic acid where sulphuric acid
catalyst is needed and heat under reflux)
(RCO)2O + CH3CH2OH
RCO2CH2CH3 + RCO2H
O
CH3 C
O
+
H
H 3C
C
O
H
H
C
C
H
H
O
H
H
H
O
C
C
O
H
H
H
C
C
H
H
H
+ CH3CO2H
Nucleophilic Addition Elimination Mechanism
δO
δ+
CH3 C
Cl
nucleophilic
addition
O
CH3 C
+
OCH2CH3
Cl H
OCH2CH3
elimination
H
CH3
CH3
O
C
OCH2CH3
H
O
C +
OCH2CH3
Cl
H
Cl
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Nucleophilic Addition Elimination Mechanism
Exam mark scheme version
Acyl chloride :Reaction with ammonia
Change in functional group: acyl chloride
Reagents: ammonia
Conditions: room temp
O
CH3
O
+ 2NH3
C
amide
Cl
RCOCl (l) + 2NH3
+ NH4Cl (s)
CH 3 C
NH2
RCONH2 (l)+ NH4Cl
Observation: white smoke of NH4Cl is given off
The nucleophile is the lone pair of electrons on the nitrogen in
the ammonia molecule.
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Acid anhydride :Reaction with ammonia
Change in functional group: Acid anhydride
Reagents: ammonia
Conditions: room temp
(RCO)2O + 2NH3
amide
RCONH2 + RCO2-NH4+
O
CH3
O
C
+ 2NH3
O
CH3
CH3
+ CH3CO2-NH4+
C
C
NH2
O
Nucleophilic Addition Elimination Mechanism
δO
δ+
CH3 C
Cl
NH2
nucleophilic
addition
O
CH3 C
+
NH2
Cl H
elimination
H
CH3
CH3
O
C
NH2
O
C +
NH2
H
NH3
NH4+
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Nucleophilic Addition Elimination Mechanism
Exam mark scheme version
Reaction with amines
Reaction with acyl chloride
O
2
NH2
+ CH3
O
C
+
NH C CH3 +
NH3 Cl
-
Cl
phenylamine
ethanoyl chloride
N-phenylethanamide
O
2 CH
3
CH2 NH2 + CH3
C
Cl
Ethylamine
ethanoyl chloride
O
CH3 CH2 NH C CH3
+ CH
+ Cl
3CH2 NH3
N-ethylethanamide
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Nucleophilic Addition Elimination Mechanism
δO
nucleophilic
O
δ+
+
addition
CH3 C
CH3 C NHCH2CH2CH3
Cl
Cl H
NHCH2CH2CH3
elimination
H
CH3
O
C
CH3
NHCH2CH2CH3
H
Cl
O
C +
NHCH2CH2CH3
H
Cl
NH2CH2CH2CH3
+ NH2CH2CH2CH3
Nucleophilic Addition Elimination Mechanism
Exam mark scheme version
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