Download carboxylic acids esters amides (R

C AR BO XY LI C ACIDS a nd the ir Der iv ative s –N ucle op hilic Acyl
su bstitut io n
- Re view t he no me n clatu re fo r t hese co mp ou nd s in you r text b oo k
-
••
R
Z
••
R
O
O
O
R
R
Cl
O
R
R
OH
R
carboxylic
acids
acid chloride
O
<
~
anhydrides
OR
R
esters
NR'2
amides (R' = H or alkyl)
the order of reactivity is the reverse (the better the leaving group, the
more reactive RCOZ is in nucleophilic acyl substitution):
O
O
O
O
Cl
O
>
>
R
O
R
R
OH
carboxylic
acids
acid chloride
R
O
>
~
anhydrides
-
Z
O
<
<
R
R
Z
the basicity of Z determines the relative stability of carboxylic acid
derivatives; thus, the order of stability is:
O
-
O–
O–
O
OR
esters
R
NR'2
amides (R' = H or alkyl)
based on this order of reactivity, more reactive acyl compounds (acid
chlorides and anhydrides) can be converted to the less reactive ones
(carboxylic acids, esters, and amides) but the reverse is usually not true.
Why?
O
R
O
Z
Nu
R
C
Nu
Z
For a reaction to
occur, Z must be a
better leaving group
than Nu
2 possible leaving groups
For mat ion o f Acid C h lo rides
Acid chlorides (or bromides) are extremely reactive. However, they are easily
prepared from carboxylic acids by two common methods:
1) Treatment with thionyl Chloride (SOCl2):
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- the poor leaving group, OH, is transformed into a better leaving group in this
process- see M echa nis m 22 .5 .
2) Treatment with oxalyl chloride (a much nicer reaction!):
cat. DMF
This reaction is believed to proceed via the mechanism below:
Why make acid chlorides? They are the fastest way to get to any other
carboxylic acid derivative, or to a number of other carbonyl compounds:
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The mechanism for all of the above reactions (except the last [Friedel-Crafts]) is
pretty much the same – a nucleophile adds to the electrophilic carbonyl group,
creating a tetrahedral intermediate. The electrons on oxygen then pop down,
expelling the good leaving group (Cl-). This type of reaction is frequently called
an addition-elimination reaction:
Acid Anh ydr id es:
O
Ph
O
O
O
Ph
benzoic anhydride
Me
O
O
Ph
acetic benzoic anhydride
Symmetrical anhydrides are typically made by mixing an acid under
dehydrating conditions. For the most part, this is a difficult reaction to perform,
and since the reactivity of anhydrides is so similar to that of acid chlorides,
anhydrides are not commonly used in synthesis.
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There are two exceptions to this statement. First, acetic anhydride is fairly easy
to prepare in bulk from dirt-cheap acetic acid, and so it is often used in place of
acetyl chloride (MeCOCl).
Second, cyclic anhydrides are fairly easily formed by heating molecules that have
two carboxylic acid functions in close proximity to high temperatures (a
dehydration reaction). A couple of examples are maleic anhydride and phthalic
anhydride:
Cyclic anhydrides have use in organic chemistry; e.g. in Diels-Alder reactions.
Again, the most commonly used anhydride is acetic anhydride. This reagent can
be used to acetylate functional groups such as alcohols and amines. Acetylation
can modify both the chemistry and biological activity of a compound. In the case
of aspirin, for example, acetylation of the relatively acidic phenol alcohol of
salicylic acid leads to a compound that doesn’t dissolve your stomach lining so
easily...
O
NH2
Me
HO
H
N
O
O
Me
Me
O
HO
acetaminophen (active
ingredient in tylenol)
So, remember...anhydrides react just about like acid chlorides.
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O
R'OH as solvent
O
HCl or H2SO4 catalyst
+ H2O
R
OH
R
OR'
-To drive the reaction to completion, excess alcohol must be used or water must be
removed as it is formed. See Mechanism 22.6.
- esterification of a carboxylic acid occurs in the presence of an acid but not in the
presence of a base (since a carboxylate ion results under basic conditions).
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performed on an ester rather than an acid. Finally, esters are very common in nature and
thus are often the final goal of synthesis
O
R
O
1. MeMgBr
OH
2. H2O
R
+ CH4
OH
MeOH/H2SO4
O
R
OH
1. MeMgBr
OMe
2. H2O
R
Me
Me
reverse of Fischer Esterification of carboxylic acids (see mechanism 22.8).
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