Transcript Chapter 1 Structure and Bonding

Chapter 17 Aldehydes and Ketones
I.
O
Properties and Names of Aldehydes and Ketones
A.
C
Nomenclature
1) Carbonyl groups are the highest priority functional group we have seen
2) Aldehydes
a) Common Names modify the name of the corresponding carboxylic acid
O
O
H3C
O
C
OH
Acetic Acid
b)
O
H3C
C
O
C
H
H
Acetaldehyde
H
C
H
Formaldehyde
Benzaldehyde
IUPAC Names treat aldehydes as derivatives of alkanes
i. Suffix –al added to the alkane name: alkane becomes alkanal
ii. The carbonyl carbon is #1, but is not numbered in the name
iii. Complex aldehydes are named as “alkanecarbaldehydes” O
O
CH3CH2CH
ClCH2CH2CH2CH
propanal
4-chlorobutanal
H
4,6-dimethylheptanal
O
H
cyclohexanecarbaldehyde
H
3)
Ketones
a) Common Names come from the 2 R groups listed in alphabetical order,
followed by “ketone”. Phenyl Ketones have common names ending in “O
O
phenone”
O
O
H3C
C
CH3
CH3CH2
dimethyl ketone
acetone
b)
H3C
C
C
C
CH3
ethyl methyl ketone
Acetophenone
Benzophenone
IUPAC Names of ketones modify the alkane name with “-one”
i. The carbonyl carbon is assigned the lowest possible number
ii. Aromatic ketones are named as aryl substituted alkanones
iii. A ketone with an aldehyde is called an “oxo-” substituent
Cl
CH3
O
O
O
H3C
C
CH3
propanone
O
O
4-chloro-6-methyl-3-heptanone 2,2-dimethyl
cyclopentanone
2-pentanone
O
O
OH
O
1-phenylethanone
O
O
COOH CH3CCH2CH CH3CCH2CH=CHCH2CCH3 H C C C
3-oxobutanal
O
H
CH3
propynal
4-formylcyclohexane
7-hydroxy-7-methyl-4-octen-2-one
carboxylic acid
Br 5-bromo-3-ethynyl
cycloheptanone
4)
Carbonyl groups named as a substitutent are called alkanoyl (or acyl) groups
O
H3C
O
C
H
ethanoylor acetyl-
5)
C
formyl-
Drawing Aldehydes and Ketones
a) Aldehyde = RCHO, Alcohol = RCOH
b) Ketone: RCH2COCH3
O
O
CH3CH2CH2CH
CH3CH2CH2CHO
B.
Butanal
O
O
CH3CH2CCH3
H
CH3CH2COCH3
2-butanone
Carbonyl Structure and Physical Properties
1)
C=O bond is similar to C=C bond
a) C(sp2)—O(sp2) s-bond overlap, with a p-p overlap for the p-bond
b) Planar group with 3 120o angles
2)
3)
4)
The C=O bond is fairly strong, 175-180 kcal/mol (ethene = 173 kcal/mol)
a) Oxygen is electronegative, so the bond is polar
b) The partially positive charged carbon is electrophilic
c) The partially negatively charged oxygen is nucleophilic and basic
d) Resonance structures:
O
O
C
C
Boiling points are higher than alkanes due to polarity
Small carbonyl compounds (< 7 C) are water soluble due to polarity (acetone)
C.
Spectroscopy
1. 1H NMR
a) Aldehyde proton is extremely deshielded, d = 9-10 ppm
i. Movement of p-electrons reinforces magnetic field
ii. d+ C increases the deshielding beyond that effect
b) Hydrogens adjacent to the aldehyde are also slightly deshielded
c) Ketones also slightly deshield adjacent hydrogens
O
RCH2
2.5
C
H
9.8
O
R2CH
2.6
C
CH3
2.0
2)
O
CH3CH
31.2 199.6
13C
NMR
a) Carbonyl carbons are observed at around 200 ppm due to d+ C
b) Adjacent carbons are somewhat effected as well
O
CH3CH2CH
5.2 36.7 201.8
O
CH3CCH3
30.2 205.1
O
CH3CCH2CH2CH3
29.3 206.6 45.2 17.5 13.5
3)
IR
a)
b)
c)
d)
e)
O
C=O stretch is intense, 1690-1750 cm-1
Aldehyde carbonyl usually around 1735 cm-1
Ketone carbonyl usually around 1715 cm-1
Conjugation reduces the wavenumber by 30-40 cm-1
Small rings increase the wavenumber
1680 cm-1
CH3
O
1745 cm-1
4)
II.
UV-Vis
a) Nonbonding oxygen lone pairs give np*
b) p-bond gives pp* transitions
c) Acetone: np* = 280 nm (e = 15), pp* = 190 nm (e = 1100)
d) Conjugation shifts the absorbances to longer wavelenth (lower E)
Preparation of Aldehydes and Ketones
A.
Oxidation of Alcohols
1) Cr(VI) reagents (like CrO3) oxidize alcohols to carbonyls
H
2) Secondary ROH gives ketones
CrO3
H3C C OH
CH3
H3C
O
C
CH3
3)
Primary ROH gives aldehydes
a) Must be done under anhydrous conditions to prevent overoxidation
b) PCC = pyridinium chlorochromate, pyridine, and CH2Cl2 conditions
PCC, py
CrO3
CH3CHO
CH3CH2OH
CH3CH3COOH
CH3CH2OH
CH2Cl2
c) Manganese dioxide (MnO2) oxidizes only allylic alcohols; it won’t
react with ordinary alcohols.
O
MnO2
HOCH2CH2CH=CHCH
HOCH2CH2CH=CHCH2OH
CHCl3
B.
Ozonolysis of Alkenes
O
CH3
1. O3
O
CH3CCH2CH2CH2CH2CH
2. Zn
C.
Hydration of Alkynes
1. Markovnikov hydration yields ketones
+
2+
H2O, H , Hg
HO
H
R
O
R C
C C
R C CH
tautomerization
H
CH3
2.
Anti-Markovnikov hydration yields aldehydes
H
R2BH
BR2
C C
R C CH
R
D.
H
H
H2O2
HO
OH tautomerization
RCH2CH
C C
-
R
H
Aryl Ketones Via Friedel Crafts Alkanoylation (Acylation)
O
CH3O
1. CH3CCl, AlCl3
CH3O
2. HCl, H2O
C
CH3
O
III. Additions to Carbonyls
A.
Three regions of Carbonyl Reactivity
1) The :O: is nucleophilic and will attack electrophiles
2) The C is electrophilic and will be attacked by nucleophiles
3) The a-C has acidic protons (NEXT CHAPTER)
O
O
C
C
CH2
CH2
O
B.
Hydrogenation
1. Catalytic Hydrogenation reduces carbonyls
OH
O
CH3CCH2CH3
2.
3.
H2
Ra Ni
CH3CHCH2CH3
Reaction is slower than for alkenes: higher H2 pressure and temp. needed
C=C bonds can be selectively hydrogenated in the presence of C=O
H2 (1 atm)
o
Pt, 25 C
O
O
C.
O
X+Y-
OX
Ionic Additions
C
C
d+
d1. Polar X—Y molecules will add to C=O
Y
2. Hydrides add 2 H atoms to the carbonyl, but won’t reduce alkenes
O
OH
NaBH4
CH3CCH2CH3
CH3CHCH2CH3
1. LiAlH4, Et2O
O
2. H+, H2O
H
OH
3.
Grignard Reagents add R, H to the carbonyl
OH
O
CH3CCH2CH3
RMgBr
THF
CH3CCH2CH3
R
4.
Milder Reagents
a) Hydrides and Grignard Reagents are strong bases. They irreversibly
add to the carbonyl
b) Less basic reagents can also add to carbonyls, but the reactions are
reversible: H2O, ROH, RSH, RHN2, etc…
c) The conditions used in the reaction of these milder bases determine
how the reaction proceeds
5.
Nucleophilic Addition-Protonation (Basic Conditions)
a) Mechanism
+
-
d
d
C O
Nu-
C O
Nu
Alkoxide ion
H OH
-
C OH + OH
Nu
b)
c)
d)
e)
f)
g)
6.
d+ dC O
Nucleophile approaches, causing C to rehybridize
p-bond electrons move to Oxygen, producing an alkoxide anion
Protonation from solvent yields the product
The new Nu—C bond has both electrons from Nu- (like in SN2)
An electron pair is the “leaving group”
Strongly basic nucleophiles typically follow this mechanism
Electrophilic Protonation-Addition (Acidic Conditions)
a) Mechanism
C OH
+
H
pKa of C=OH is -8
b)
c)
d)
C OH
Nu
C OH
Nu
C=O is a weak base (strong acid) so small, but reactive, amount present
Nu attacks carbon electrophile to give product. This removes
intermediate and shifts the equilibrium to the right.
Weakly basic nucleophiles typically follow this mechanism. Strongly
basic nucleophiles would just get protonated and couldn’t react.