Post on 25-Jan-2016
description
transcript
Chapter 16
Aldehydes and Aldehydes and KetonesKetones
Structure
The functional group of an aldehydealdehyde is a carbonyl group bonded to a hydrogen atom
The functional group of a ketoneketone is a carbonyl group bonded to two carbons
CH3CHO
HCHO
CH3CCH3
O
Propanone(Acetone)
Ethanal(Acetaldehyde)
Methanal(Formaldehyde)
Nomenclature
IUPAC names for aldehydes to name an aldehyde, change the suffix -ee of the
parent alkane to -alal the aldehyde carbon must be carbon-1 for unsaturated aldehydesunsaturated aldehydes, indicate the presence of
a carbon-carbon double bond and an aldehyde by changing the ending of the parent alkane from -aneane to -enalenal; show the location of the carbon-carbon double bond by the number of its first carbon
Nomenclature
the IUPAC system retains common names for some aldehydes, including these three
3-Methylbutanal 2-Propenal(Acrolein)
Hexanal
12
34H
O
H
O1
23
45
6
123
H
O
CHO
H
OCHO
OCH3
OHtrans-3-Phenyl-2-propenal
(Cinnamaldehyde; inoil of cinnamon)
Benzaldehyde(in almonds)
Vanillin(from vanilla
beans)
Nomenclature
IUPAC names for ketones parent = longest chain that contains the carbonyl indicate with parent alkane as -oneone carbonyl carbon gets the smaller number the IUPAC retains the common name acetone for
2-propanone
O
Acetone 2-Methylcyclohexanone5-Methyl-3-hexanone
OO
12
34
56
12
Nomenclature To name an aldehyde or ketone that also
contains an -OH or -NH2 group give the carbonyl carbon the lower number indicate an -OH substituent by hydroxy-hydroxy-, and an -NH2
substituent by amino-amino- hydroxy and amino substituents are numbered and
alphabetized along with other substituents
O
H
OOH
NH2
3-Hydroxy-4-methylpentanal 3-Amino-4-ethyl-2-hexanone
1345 12346
Nomenclature
Common names derived from the common name of the
corresponding carboxylic acid; drop the word "acidacid" and change the suffix -icic or -oicoic to -aldehydealdehyde
name each alkyl or aryl group bonded to the carbonyl carbon as a separate word, followed by the word "ketoneketone”;
O
CH3CH
O
CH3COH
Acetaldehyde Acetic acid Ethyl isopropyl ketoneMethyl ethyl ketone
OO
Physical Properties
A C=O bond is polar, with oxygen bearing a partial negative charge and carbon bearing a partial positive charge
Physical Properties
in liquid aldehydes and ketones, the intermolecular attractions are polar
no hydrogen bonding is possible between aldehyde or ketone molecules
aldehydes and ketones have lower boiling points than alcohols and carboxylic acids, compounds in which there is hydrogen bonding between molecules
Physical Properties
formaldehyde, acetaldehyde, and acetone are infinitely soluble in water
aldehydes and ketones become less soluble in water as the hydrocarbon portion of the molecule increases in size,
CH3CH2CH2CH2CH3CH3CH2CH2CHO
CH3CH2CH2CH2OHCH3CH2COOH
CH3CH2COCH3
CH3CH2OCH2CH3pentanebutanal2-butanone1-butanolpropanoic acid
Name Structural FormulaMolecular
Weight (amu)
72727274
72
367680
117
141
bp(°C)
diethyl ether 74 34
Oxidation Aldehydes are oxidized to carboxylic acids by
a variety of oxidizing agents, including potassium dichromate
liquid aldehydes are sensitive to oxidation by O2
H
OK2Cr2O7
H2SO4OH
O
Hexanal Hexanoic acid
CH
O
O2
COH
O
Benzoic acidBenzaldehyde
+
Oxidation
Ketones resist oxidation by most oxidizing agents, including potassium dichromate and molecular oxygen
Reduction
The carbonyl group of an aldehyde or ketone can be reduced to an -CHOH group by hydrogen in the presence of a metal catalyst
H2
transition metal catalyst+H
O
PentanalOH
1-Pentanol
H2
transition metal catalyst
+O
Cyclopentanone
OH
Cyclopentanol
Reduction
The most common laboratory reagent for the reduction of an aldehyde or ketone is sodium borohydride, NaBHNaBH44 hydrogen in the form of hydride ion, H:H:--
in a reduction by sodium borohydride, hydride ion adds to the partially positive carbonyl carbon which leaves a negative charge on the carbonyl oxygen
reaction of this intermediate with aqueous acid gives the alcohol
Reduction
does not affect a carbon-carbon double bond
HCO
1. NaBH4
2. H2O
CH2OH
Cinnamaldehyde Cinnamyl alcohol
O NaBH4O-
HH3O+ O-H
H
H - C O H C O - H3O+
H C O-H: +
: : :: :
::
Hydrideion
Reduction
In biological systems, the agent for the reduction of aldehydes and ketones is NADH (Section 26.3) this reducing agent also delivers a hydride ion reduction of pyruvate, the end product of glycolysis,
by NADH gives lactate
CH3-C-COO-O
NADH CH3-C-COO-
H
O-
H3O+
CH3-C-COO-
H
O-H
Pyruvate Lactate
Addition of Alcohols
Addition of a molecule of alcohol to the carbonyl group of an aldehyde or ketone forms a hemiacetalhemiacetal (a half-acetal) the functional group of a hemiacetal is a carbon
bonded to one -OH group and one -OR group in forming a hemiacetal, H of the alcohol adds to
the carbonyl oxygen and OR adds to the carbonyl carbon
CH
OO-CH2CH3
HC OCH2CH3
H
O-H+
Benzaldehyde Ethanol A hemiacetal
Addition of Alcohols
hemiacetals are generally unstable and are only minor components of an equilibrium mixture
If a five- or six-membered ring can form, the compound exists almost entirely in a cyclic hemiacetal form
H
O
O-HC
O O
H
H
O O-H
H
4-Hydroxypentanal A cyclic hemiacetal
123
45
1345
redraw to show the -OH and -CHO close
to each other2
Addition of Alcohols
A hemiacetal can react further with an alcohol to form an acetalacetal plus water this reaction is acid catalyzed the functional group of an acetal is a carbon
bonded to two -OR groups
C OCH2CH3
H
O-HOCH2CH3
H H+
C OCH2CH3
H
OCH2CH3
H2O
A hemiacetal(from benzaldehyde)
Ethanol
+ +
An acetal
Addition of Alcohols
all steps are reversibleLe Chatelier's principle
to drive it to the right, we either use a large excess of alcohol or remove water from the equilibrium mixture
to drive it to the left, we use a large excess of water
OCH2CH3
O-HOCH2CH3
HH+
OCH2CH3
OCH2CH3H2O
An acetalA hemiacetal(from cyclohexanone)
Ethanol
+ +
Keto-Enol Tautomerism
A carbon atom adjacent to a carbonyl group is called an -carbon-carbon, and a hydrogen atom bonded to it is called an -hydrogen-hydrogen
-carbons
-hydrogens
CH3-C-CH2-CH3
O
Keto-Enol Tautomerism
A carbonyl compound that has a hydrogen on an -carbon is in equilibrium with a constitutional isomer called an enolenol
in a keto-enol equilibrium, the keto form generally predominates
CH3-C-CH3
OCH3-C=CH2
OH
Acetone(keto form)
Acetone(enol form)
Keto-Enol Tautomerism
example:example: draw structural formulas for the two enol forms for each ketone
(a)
(b)
O
O
Keto-Enol Tautomerism
example:example: draw structural formulas for the two enol forms for each ketone
solution:solution:
(a)
(b)
O
O
OH OH
OH OH
End End Chapter 17Chapter 17
Aldehydes and Ketones