Alcohol

In general usage, alcohol (from Arabic al-khwl الكحول, or al-ghawl الغول) refers almost always to ethanol, also known as grain alcohol, and often to any beverage that contains ethanol (see alcoholic beverage). This sense underlies the term alcoholism (addiction to alcohol). Other forms of alcohol are usually described with a clarifying adjective, as in isopropyl alcohol or by the suffix -ol, as in isopropanol.

Chemistry of alcohols

Preparation

Laboratory

There are three common methods:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• From alkyl halides: react with aqueous NaOH or KOH (mainly 1° alcohols).

:R-Br + KOH → R-OH + KBr

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• From aldehydes or ketones: reduction with sodium borohydride or lithium aluminium hydride.

:R-CHO - → R-OH

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• From alkenes: an acid catalysed hydration reaction using concentrated sulfuric acid as a catalyst (gives usually 2° or 3° alcohols).

:C2H4 + H2SO4 (l) → C2H5-HSO4

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:C2H5-HSO4 + H2O → C2H5OH + H2SO4

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The formation of a secondary alcohol via the last two methods is shown:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Industrial

• Fermentation: using glucose produced from sugar from the hydrolysis of starch, in the presence of yeast and temperature of <37°C to produce ethanol.

:C12H22O11 → C6H12O6 + C6H12O6

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:Invertase → glucose + fructose

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:C6H12O6 + H2O → C2H5OH + CO2

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:Glucose → zymase + ethanol

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• Direct hydration: using ethene or other alkenes from cracking of fractions of distilled crude oil. Uses a catalyst of phosphoric acid under high temperature and pressure.
• Methanol from water gas: It is manufactured from synthesis gas, where CO + 2 H₂ are combined to produce methanol using a Cu, ZnO and Al₂O₃ catalyst at 250°C and a pressure of 50-100 atm.

: + H2O (g) → CH3OH

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Reactions

See the physical and chemical properties section above for a general overview.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Deprotonation

Alcohols can behave as weak acids, undergoing deprotonation. The deprotonation reaction to produce an alkoxide salt is either performed with a strong base such as sodium hydride or n-butyllithium, or with sodium or potassium metal.

Related Topics:
Alkoxide - Sodium hydride - ''n''-butyllithium

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

: 2 R-OH + 2 NaH → 2 R-O-Na+ + H2↑

Related Topics:
NaH - H₂

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

: 2 R-OH + 2Na → 2R-O−Na+

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

: e.g. 2 CH3CH2-OH + 2 Na → 2 CH3-CH2-O−Na+

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Water is similar in pKa to many alcohols, so with sodium hydroxide there is an equilibrium set up which usually lies to the left:

Related Topics:
PK_a - Sodium hydroxide

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

: R-OH + NaOH R-O-Na+ + H2O (equilibrium to the left)

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Nucleophilic substitution

The OH group is not a good leaving group in nucleophilic substitution reactions, so neutral alcohols do not react in such reactions. However if the oxygen is first protonated to give R−OH2+, the leaving group (water) is much more stable, and nucleophilic substitution can take place. For instance, tertiary alcohols react with hydrochloric acid to produce tertiary alkyl halides, where the hydroxyl group is replaced by a chlorine atom. If primary or secondary alcohols are to be reacted with hydrochloric acid, an activator such as zinc chloride is needed. Alternatively the conversion may be performed directly using thionyl chloride.

Related Topics:
OH - Leaving group - Nucleophilic substitution - Water - Hydrochloric acid - Alkyl halide - Hydroxyl group - Chlorine - Zinc chloride - Thionyl chloride

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Alcohols may likewise be converted to alkyl bromides using hydrobromic acid or phosphorus tribromide, for example:

Related Topics:
Hydrobromic acid - Phosphorus tribromide

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

: 3 R-OH + PBr3 → 3 RBr + H3PO3

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

In the Barton-McCombie deoxygenation an alcohol is deoxygenated to an alkane with tributyltin hydride or a trimethylborane-water complex in a radical substitution reaction.

Related Topics:
Barton-McCombie deoxygenation - Alkane - Tributyltin hydride - Trimethylborane - Radical substitution

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Dehydration

Alcohols are themselves nucleophilic, so R−OH2+ can react with ROH to produce ethers and water, although this reaction is rarely used except in the manufacture of diethyl ether.

Related Topics:
Ether - Diethyl ether

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

More useful is the E1 elimination reaction of alcohols to produce alkenes. The reaction generally obeys Zaitsev's Rule, which states that the most stable (usually the most substituted) alkene is formed. Tertiary alcohols eliminate easily at just above room temperature, but primary alcohols requre a higher temperature.

Related Topics:
Elimination reaction - Alkene - Zaitsev's Rule

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

This is a diagram of acid catalysed dehydration of ethanol to produce ethene:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Esterification

To form an ester from an alcohol and a carboxylic acid the reaction, known as "Fischer esterification", is usually performed at reflux with a catalyst of concentrated sulfuric acid:

Related Topics:
Ester - Alcohol - Carboxylic acid - Reflux - Catalyst - Sulfuric acid

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

: R-OH + R'-COOH Leftrightarrow R'-COOR + H2O

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

In order to drive the equilibrium to the right and produce a good yield of ester, water is usually removed, either by an excess of H2SO4 or by using a Dean-Stark apparatus. Esters may also be prepared by reaction of the alcohol with an acid chloride in the presence of a base such as pyridine.

Related Topics:
Yield - Dean-Stark apparatus - Acid chloride - Pyridine

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Other types of ester are prepared similarly- for example p-toluenesulfonate (tosylate) esters are made by reaction of the alcohol with p-toluenesulfonyl chloride in pyridine.

Related Topics:
P-toluenesulfonate - Toluenesulfonyl

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Oxidation

Primary alcohols generally give aldehydes or carboxylic acids upon oxidation, while secondary alcohols give ketones. Tertiary alcohols resist oxidation. Traditionally strong oxidants such as dichromate ion or potassium permanganate are used, under acidic conditions, for example:

Related Topics:
Aldehyde - Carboxylic acid - Ketone - Oxidants - Dichromate - Potassium permanganate

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

3 CH3-CH(-OH)-CH3 + K2Cr2O7 + 4 H2SO4 → 3 CH3-C(=O)-CH3 + Cr2(SO4)3 + K2SO4 + 7 H2O

Related Topics:
CH₃-CH(-OH)-CH₃ - K₂Cr₂O₇ - H₂SO₄ - CH₃-C(=O)-CH₃ - H₂O

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Frequently in aldehyde preparations these reagents cause a problem of over-oxidation to the carboxylic acid- to avoid this other reagents such as PCC and o-iodoxybenzoic acid (IBX), or methods such as Swern oxidation are now preferred.

Related Topics:
PCC - Swern oxidation

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Alcohols with a methyl group attached to the alcohol carbon can also undergo a haloform reaction (such as the iodoform reaction) in the presence of the halogen and a base such as sodium hydroxide.

Related Topics:
Methyl - Haloform reaction - Iodoform reaction - Halogen - Sodium hydroxide

~ ~ ~ ~ ~ ~ ~ ~ ~ ~