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Synthesis and Structure of Alcohols
Alcohols can be considered organic analogues of water
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Alcohols with the hydroxyl bound directly to an aromatic (benzene) ring are called phenols
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Page 1
Nomenclature of Alcohols
(Normally any compound’s name which ends in –ol is an alcohol of some sort)
IUPAC rules that:
(1) Name the longest carbon chain bearing the –OH group
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(2) Number the longest chain starting at the end nearest the –OH group, and designate a number for the –OH
group
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(3) Name the remaining substituents and their numbers as for alkanes and alkenes
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(4) Cyclic alcohols have the prefix cyclo-, and the hydroxyl group is deemed to be on C-1
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Numbering
gives the hydroxyl group the lowest possible number
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Page 3
(6) If the hydroxyl group is only a minor part of the structure, it may be named as a hydroxy- substituent
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They are named as for alcohols except the suffix –diol is used, and two numbers are required to locate the –OH’s
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These terms are non-IUPAC though
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Page 4
Solubility of Alcohols
The hydroxyl groups in alcohols can form hydrogen bonds with water, and many low molecular weight alcohols
are miscible with water
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(E
...
NaCl is partially
soluble in Ethanol)
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Alcohols will also dissolve in hydrocarbon solvents
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Page 5
Acidity of Alcohols and Phenols
Just like water, the hydroxyl groups in alcohols are weakly acidic – strong bases can generate alkoxide ions
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Alcohol
pKa
Methanol
Ethanol
2-chloroethanol
2,2,2-trifluoroethanol
t-butylalcohol
cyclohexanol
phenol
CH3-OH
C2H5-OH
Cl-C2H4-OH
CF3CH2-OH
(CH3)3C-OH
C6H11-OH
C6H5-OH
15
...
9
14
...
4
19
...
0
10
...
7
4
...
2
Notice that electron withdrawing groups on an alcohol increase the acidity by stabilizing the alkoxide formed
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Page 6
Acidity of Phenols
Cyclohexanol and phenol are similar in structure, yet their acidities are very different
...
O-H
O-
+ H2O
+ H3O+
O-H
O-
+ H2O
+ H3O+
Ka = 10
-18
Ka = 10
-10
The phenoxide ion has resonance stabilization since the negative charge can be delocalised over 4 atoms (3 carbons
and 1 oxygen), making it more stable
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Ch10 Alcohols; Struct + synth (landscape)
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Page 8
Syntheses of Alcohols
Previously seen:
Nucleophilic substitution of alkyl halide with HOAcid catalyzed hydration of alkenes
Oxymercuration-demercuration
Hydroboration-oxidation (anti-Markovnikov)
Hydroxylation gives diols (syn – OsO4 :
anti – epoxidation-hydrolysis)
Acetylides and carbonyl compounds
Organometallic Reagents for Alcohol Synthesis
When a compound has a covalent bond between a carbon and a metal, it is called an organometallic compound
...
A carbon metal bond is polarized with significant negative charge on the carbon because metals are so
electropositive
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Ch10 Alcohols; Struct + synth (landscape)
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They are made via the reaction of metallic Mg with the corresponding alkyl halide, usually in ether type solvents
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Organolithium Reagents
These are also formed via action of the metal with an alkyl halide
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Ch10 Alcohols; Struct + synth (landscape)
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Recall, the carbonyl group is strongly polarized, with the oxygen bearing partial negative charge and the carbon
bearing partial positive charge – the carbon is electrophilic, and therefore attacked by nucleophiles
...
A Grignard (or alkyl lithium) can function as this nucleophile
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Ch10 Alcohols; Struct + synth (landscape)
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Aldehydes have one alkyl group and one hydrogen bound to the carbonyl
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E
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Ch10 Alcohols; Struct + synth (landscape)
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Formation of Tertiary Alcohols
This can be achieved by reaction of a Grignard with a ketone
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Page 13
Acid Derivatives
Acid chlorides and esters are derivatives of carboxylic acids, where the –OH group is replaced either by a chlorine
atom or an alkoxy group
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Ch10 Alcohols; Struct + synth (landscape)
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The leaving group is expelled (either chloride or alkoxide) as the carbonyl group is reformed, and a ketone is
produced
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Ch10 Alcohols; Struct + synth (landscape)
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E
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Ch10 Alcohols; Struct + synth (landscape)
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After protonation, the final product is a primary alcohol with two additional carbon atoms
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Page 17
Other Selected Organometallic Reactions
Corey-House Reaction
This is a good synthetic way to join two alkyl groups together
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The lithium dialkylcuprate is formed using the alkylithium reagent and copper (I) iodide
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Ch10 Alcohols; Struct + synth (landscape)
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Acidic Compounds
Organometallic compounds react with water (moisture)
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Often small amounts of water (humid air) can destroy hard earned organometallic compounds
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(R-X R-H)
The use of Grignards and deuterium oxide (D2O) is a useful way to incorporate Deuterium into an organic
molecule
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Incompatibility
Besides –OH groups, other acidic groups like N-H and S-H cannot be tolerated by organometallics
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These groups are considered incompatible with organometallics
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Other Reductions of Alkyl Halides
A combination of metals and acids can be used to reduce alkyl halides to alkanes
...
g
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Page 20
Hydride Reductions
Complex hydrides are the source of hydride ions, and the two most commonly used reagents are sodium
borohydride and lithium aluminum hydride
...
Lithium aluminum hydride (LiAlH4) is a strong reducing agent, and will reduce alkyl halides to alkanes
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Ch10 Alcohols; Struct + synth (landscape)
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The conversion of ketones to alcohols involves the addition of H2 across the C=O bond – a reduction
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Ch10 Alcohols; Struct + synth (landscape)
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Sodium borohydride will not reduce esters or acids
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These hydride sources do not reduce C=C double bonds
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Page 23
Catalytic Hydrogenation
Carbonyls can be reduced to alcohols by the addition of hydrogen across the double bond
...
H
H2, Ni
O
OH
However, C=C double bonds are reduced quicker than C=O double bonds
...
They are named using the suffix –thiol, and as a substituent as mercapto-
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Ch10 Alcohols; Struct + synth (landscape)
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Thiols (unlike alcohols) are easily oxidized to form disulfides
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Ch10 Alcohols; Struct + synth (landscape)