Carboxylic acids Flashcards
Carboxylic acid
COOH (image)
Carboxylic acids are
Dimeric
Dimeric
Consisting of two similar molecules that have been linked together
Low Pka
strong acid
high pka
weak acid
strength of alkanes as an acid
very weak acid
are carboxylic acids strong or weak acids
most are weak acids
pKa table
draw table out
electronegativity
a measure of the ability of an atom in a chemical compound to attract electrons
inductive effects
stabilize charge through sigma bonds
electron-withdrawing substituents increase
acidity of nearby atom
electron withdrawing groups effect increases with ____ and decreases with ______
electronegativity, distance
Rank chlorine, iodine, fluorine and bromine in order of increasing electronegativity
F > Cl > Br > I
A carboxylic acid with F as a substituent is ___ acidic than one without
more
two methods of preparing carboxylic acids
1) oxidation in aqueous conditions 2) addition of Grignard or alkyl lithium reagents to CO2
carboxylic acids ____ undergo nucleophilic substitution with negatively charged nucleophiles
do NOT
LiAlH4 will reduce _____ to _____
carboxylic acids, primary alcohols
reduction using LiAlH4
1) deprotonation
2) addition of hydride
3) elimination
4) addition of hydride
5) protonation
(Image)
Acid chloride
Image
Anhydride
Image
Ester
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Amide
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Nucleophilic substitution
A type of substitution reaction in which a nucleophile is attracted to an electron-deficient centre or atom, where it donates a pair of electrons to form a new covalent bond.
Addition elimination mechanism
1) add to carbonyl
2) elimination of leaving group
Nucleophilic acyl substitution
1) Nucleophilic addition
2) elimination of the leaving group and reformation of the carbonyl (Image)
Golden rule of acid base reactions
stronger acids and stronger base should react to give a weaker acid and a weaker base
Four derivatives ranked by reactivity
Acid Chloride > anhydride > ester > amide
Two nucleophilic acyl substitutions that don’t work
Anhydride into acid chloride ester into anhydride
Conversion of carboxylic acids to acid halides
1) chloride is a bad nucleophile
2) leaving group must be worse than chloride
3) leaving group disintegrates making the reaction irreversible
4) requires SOCl2, HCl, and SO2 gas
Mechanism of carboxylic acids to acid halides
1) attack on thionyl chloride
2) additon of Cl- to carbonyl carbon
3) elimination of SO2Cl
4) deprotonation
Reaction of carboxylic acid with alcohols to make esters
Requires an acid catalyst (H2SO4) and an alcohol
Esterfication
The reaction of an alcohol with a carboxylic acid to produce an ester and water (Image)
Using an excess of alcohol drives the esterification equilibrium towards the
Ester product
Equal concentrations of alcohol and acid
65% ester
10 fold excess of alcohol to acid
97% ester
100 fold excess of alcohol to acid
99% ester
Mechanism of esterfication
Image
Backwards reaction of esterfication
Hydrolysis
Mechanism of hydrolysis
Image
Ester hydrolysis is best done under ____ conditions
basic
Amides are the product of a net condensation reaction between a
carboxylic acid and an amine
Consequence of partial double bond character
Restricted rotation
For amides, interconversion of conformers is slow due to
significant double bond character of C-N bond
Amides react with acid at the
Oxygen
Amine N-H bonds are less acidic than
Amide N-H bonds
Brute force method of amide synthesis from acids
Wrong method. Carboxylic acids and amines form salts through acid base reactions
Pyrolysis
The decomposition of organic matter by heat
Right way to prepare amides from acids
Combination of amines and carboxylic acids int he presence of a dehydrating reagent such as DCC
DCC
Image
Mechanism of preparing amines from carboxylic acids
1) deprotonation of acid
2) attack on DCC
3) addition
4) proton transfer
5)elimination
Basic hydrolysis of amides requires
Strong base and heat
Mechanism of basic amide hydrolysis
1) Addition
2) Elimination
3) Deprotonation
Lactams
cyclic amides
Best condones for hydrolysis of amides
Acidic conditions, need more mild conditions than the conditions required for basic conditions
Mechanism of acid hydrolysis of amide
Image
What explains high reactivity
Resonance forms
Non aromatic compounds are ____ reactive than aromatic compounds
More
Strecker reaction
Image
Hydrolysis of nitriles to carboxylic acids requires
Aqueous acid
Mechanism of nitrile hydrolysis
Image
Under gentle reaction conditions, the hydrolysis of nitriles stops at
the amide
In acyl halide reactivity _____ loses every time
Chloride
Features of lithium tri-t-butoxy aluminium hydride
- less reactive than LiAlH4
- cannot reduce carboxylic acids
- Use of one equivalent reduces acid chlorides to aldehydes
-Catalytic hydrogenation also works
Reduction of acyl halides
Image
Mechanism of reception of acid halides
1) addition of hydride
2) elimination of halides
3) deprotonation
4) acid workup
Hydrolysis of esters under basic conditions requires
NaOH and acid workup
Mechanism of ester hydrolysis
Image
Cyclic esters can undergo
basic hydrolysis
Saponification
the reaction between a fat and a strong base to produce glycerol and the salt of a fatty acid (soap)
Transesterification
The process that transforms one ester to another when an alcohol acts as a nucleophile and displaces the alkoxy group on an ester.
(Image)
Mechanism of transesterfication under acidic conditions
1) protonation
2) addition
3) deprotonation
4) protonation
5) elimination
6) deprotonation
Mechanism of transesterfication
1) nucleophilic addition
2) elimination
Facts about acidic transesterfication
All steps in equilibrium
equilibrium favours whichever alcohol is present in greater concentration
Lactone
Image
DiBal
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Ester to aldehyde
Mechanism of reduction of esters to aldehydes using DiBal
1) Coordination of oxygen to Lewis acid aluminium
2) addition of hydride
3) addition of mild acid workup with warming
Reduction of nitriles with DIBal
Image
Why is an Al-O bond so strong
Electronegativity and charge differences
Mechanism reduction of amides to amines using LiAlH4
1) addition of hydride
2) elimination
3) addition of hydride
image
Mechanism of reduction of nitriles to primary amines with LiALH4
1) addition to nitrile
2) addition to C=N
3) mild acid workup
factors associated with acidity
inductive effects
resonance delocalization
electronegativity of neighbouring groups
esters are ___ than aldehydes or ketones
less
enolates are most nucleophilic at
carbon
enolate
intermediate
stabilized by resonance
enol+ base—– enolate
where does the negative charge on enolate sit
the oxygen
enolate formation requires the C-H bond align parallel to
carbonyl bond
enolizable
must have a C-H on the alpha carbon
keto-enol tautomerism via enolates
Break C-H
Form O-H
Break C-O
Form C-C
Form O-H
Deuterium labeling
Image
Racemate
a mixture containig equal amounts of two enantiomers
Chiral centres on enolates alpha carbons undergo
racemization
Cause of racemates
addition is able to occur at the top and bottom of the attack site
Chiral
a molecule that is not superimposable on its mirror image
epimerzation
the reversible interconversion of epimers
Epimer
Diasteromers that differ at only one chiral centre
Imgae
When do epimers occur
When the alpha carbon is chiral and there is more than one chiral centre and it is treated with a base
Facts about halogenation of enolates
Non selective under basic conditions
hard to control under basic conditions
multiple halogenation can occur
acid catalyzed halogenation of ketones
can halogenate only one or two a-H’s
use acetic acid as solvent and catalyst Image
Halogenation of enolates
Image