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Alcohols - Organic Chemistry Tutor

Alcohols are the family of compounds that contain one or more hydroxyl (-OH) groups.

After the nature of the hydrocarbons radical (the radical connected to the –OH group), you can distinguish several types of alcohol:
- if the radical derives from a saturated hydrocarbon (acyclic or cyclic), the respective alcohol is a saturated alcohol (for example: ethanol CH3-CH2-OH)
- if the radical has a double bound, the respective alcohol is a unsaturated alcohol (for example: allelic alcohol CH2=CH-CH2OH)
- if the radical contains an aromatic nucleus, the respective alcohol is a aromatic alcohol (for example: benzilic alcohol C6H6-CH2OH)

After the number of hydroxyl groups in the molecule, you can differentiate 2 types of alcohols: mono hydroxyls (for example ethanol C2H5OH) and poly hydroxyls (for example: propane trill CH2OH-CHOH-CH2OH). The alcohols containing 2 hydroxyl groups connected by different carbon atoms are called glycols, for example: CH2OH-CH2OH.

Structure and nomenclature

From the mono hydroxyls alcohols formula, R-OH, you can observe that they can be considered both derivates of hydrocarbons (by replacing a hydrogen atom with a hydroxyl group -OH) and water (H2O) derivates (the result of the substitution of a hydrogen atom by an organic radical). Alcohols contain a group of –OH bound to a carbon atom through one valence. As a proof, if a alcohol is treated with hydrochloric acid the result will be water and a halogenated derivate:

R-OH (alcohol) + HCl H2O + R-Cl (halogenated derivate)

Although the rational denomination of alcohols is formed after the showed rule, by adding the suffix ‘ol’ (or the prefix “hydrox”) at the basic name of the hydrocarbon, the old nomenclature is still used (the name of the alcohols is indicated by the association of the term alcohol with the name of the respective radical). For example, ethanol is also named ethylic alcohol and propanol is also called propilic alcohol.

The denomination of an alcohol must take into the consideration the position of the carbon atom (the one connected to the hydroxyl) and the existence of isomers. For example, taking into consideration the existence of 2 propanols, they are designated as 1-propanol and 2-propanol:

CH3-CH2-CH2OH (1-propanol)
CH3-CH(OH)-CH3 (2-propanol)

The classification of mono hydroxyl alcohols

The mono hydroxyl alcohols can be classified after the primary/secondary/tertiary carbon atom connected to the hydroxyl, in:
- primary alcohols
- secondary alcohols
- tertiary alcohols

For example, ethanol CH3-CH2-OH, is called a primary alcohol; 2-propanol CH3-CH(OH)-CH3 is a secondary alcohol and 2-methyl-2-propanol CH3-C(CH3)OH-CH3 is a tertiary alcohol.

Obtaining methods


1. From alkanes

CH4 + O2 CH3-OH (methylic alcohol)

2. Water addition at alkenes (hydration)

R-CH=CH2 + H2O R-CH-OH-CH3

Examples:
CH3-CH2-CH2=CH2 + H2O CH3-CH2-CHOH-CH2 (2-hidroxobutane)
CH3-CH=CH-CH3 + H2O CH3-CHOH-CH2-CH3 (2hidroxobutane)

3. From halogenated derivates

R-X + H2O R-OH + HX

Examples:
CH3-CH2-Br + H2O CH3-CH2-OH + HBr (ethylic alcohol)
Cl-CH2-CHCl-CH2-Cl + 3H2O OH-CH2-CHOH-CH2-OH + 3HCl (glycerin)

4. From carboxylic compounds (hydrogenation)

R-C=O-R' + H2 R-CHOH-R' (alcohol)

5. From amines

R-NH2 + HNO2 H2O + N2 + R-OH (alcohol)

6. From esters through hydrolyze

R-C=O-OR' + H2O R-COOH + R'-OH (reversible reaction)

Physical properties

Alcohols, at the regular temperature, are colorless, liquid or solid substances. Their boillind points are much higher than the respective hydrocarbons, even then other similar organic combinations. These have a distinctive behavior which is due the fact that, in a liquid state, the alcohols form molecular associations, meaning that they are not found in the R-OH form, but more alcohols molecules are connected to each other through hydrogen bounds, so that they form (R-OH)x associations. This hydrogen bounds are formed between the hydrogen atoms and the oxygen atom (belonging to another molecule); they are caused by the polarity of the hydroxyl group. In a vaporous state, the bounds within alcohol molecules undo, the alcohol remaining under an unassociated form. The primary alcohols boiling points are higher than the secondary alcohols’ points (isomers with them), and the secondary ones boil at higher temperatures than the tertiary ones.

The density of alcohols is, in general, smaller than the water’s. This fact is due to the screened action of the alkyl radicals over the OH group. This action is more accentuated at tertiary alcohols. The solubility of alcohols in water diminishes as the number of carbon atoms in the molecue grow. Methanol, ethanol and propanol mix in any quantities with water. They are good solvents for some organic compounds. The superior alcohols are insoluble in water and have the property of protecting the substances (which they cover) from humidity.

Chemical reactions


1. The substitution of H (the alcohols acidity)

R-OH + Na R-ONa (natrium alcoxid) + H2
Atention! The alcohols do not react with NaOX, fenoxizi and carboxils

2.The reaction with the inorganic acids

R-OH + HX R-X + H2O

Examples:
CH3-OH + HCl CH3-Cl + H2O
CH3-CH2-OH + HBr CH3-CH2-Br + H2O

3. The reaction with anorganic acids

R-OH + R'-COOH R'-C=O-OR + H2O (equilibrated and reversible reaction – ester is formed)

4. The reaction with PX5 and PX3

R-OH + PX5 R-X + POX3 + HX
R-OH + PX3 R-X + POX + HX

5. Alcohol + alcohol

R-OH + R'-OH R-O-R' + R'-O-R'

Examples:
CH3-OH + CH3-OH CH3-O-CH3 + H2O (dimethyl)
CH3-CH2-OH + CH3-CH2-OH CH3-CH2-O-CH2-CH3 + H2O (ethylic ether)

6. The elimination reaction (dehydration)

CH3-CH2-OH CH2=CH2 + H2O (ethane)

7. Oxidation reaction

a) Complete (combustion)
CxHyOz + O2 CO2 + H2O + Q

b) Mild oxidation (K2Cr2O7/H2SO4)
R-CH2-OH + [O] R-CH=O + H2O (aldehyde)
R-CHOH-R' + [O] R-C=O-R' + H2O (acetone)

c) Energy oxidation (KMnO4, H2SO4, t degrees)
R-CH2-OH + [O] R-C=O-OH + H2O (carboxylic alcohol)

Examples:
CH3-CHOH-CH3 + 5[O] CO2 + 2H2O + CH3-COOH
CH3-CHOH-CH2-CH3 + [O] CO2 + H2O + CH3-CH2-COOH

d) Oxidation with CuO at t degrees
R-CH2-OH + CuO R-CH=O + Cu + H2O
R-CHOH-R' + CuO R-CR'=O + H2O + Cu

Examples:
CH3-CH2-CH2-OH + CuO CH3-CH2-CH=O + Cu + H2O

Use

» methanol is used for obtaining formaldehydes, plastic masses, synthetic gasoline and coloring matter
» ethanol is used for alcoholic drinks and as a solvent in the industry of lacquers and paint, also in the pharmaceutical industry
» glycerin is used for obtaining dynamite and isolating material; it is also used as a medicine

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