Structure and Properties of Alcohols

 

This is part of the HSC Chemistry course under the topic Alcohols.

HSC Chemistry Syllabus

  • investigate the structural formulae, properties and functional group including: – primary – secondary – tertiary alcohols

  • explain the properties within and between the homologous series of alcohols with reference to the intermolecular and intramolecular bonding present – boiling and melting point – solubility in water – molar enthalpy of combustion

Alcohols: Structure, Nomenclature, Boiling Point and Solubility

This video introduces students to the organic functional group – alcohols. Alcohols have different structure, nomenclature and physical properties, such as melting and boiling point and aqueous solubility, when compared to hydrocarbons.

 

 

Structure of Alcohol

  • Alcohols are classified as organic molecules which contains a –OH group attached to a sp3 hybridised carbon atom (carbon with only C–C and C–H bonds).

 

There are three types of alcohols (see below). ‘R’ is a generic representation of an alkyl (carbon) group. The three types of alcohols share similar properties due to the presence of the same functional group but also differ slightly in their physical as well as chemical properties because the nature of connectivity is different.

 

Methanol (primary alcohol)

 

  • Methanol (CH3OH) does not fit in any of the three types of alcohols but it is commonly treated as a primary alcohol because it has a similar reactivity properties.

 

Alcohol Nomenclature

  • In IUPAC nomenclature, the alcohol functional group is assigned higher priority than alkanes, alkenes and alkynes.
  • Alcohol molecules have names ending with the suffix ‘-ol’

 

3-methyl-2-buten-1-ol

(primary alcohol)

2-butyn-1-ol

(primary alcohol)

2-cyclohexen-1-ol

(secondary alcohol)

  • When assigning numbers for their positions along the longest carbon chain, the alcohol functional group is always given the lowest possible number compared to alkenes and alkynes.
  • Carboxylic acids, despite containing the ‘–OH’ group, are not considered as alcohols as they have very different reactivity. Structurally, –OH group in carboxylic acids is attached to a sp2 hybridised carbon.
  •  When demonstrating bonding and structure of alcohol, remember to explicitly show the O–H bond (this is often not shown)

Generic structural formula for a

carboxylic acid

Fully expanded structure of methanol

 

Properties of Alcohols

Polarity & Solubility

  • Alcohol molecules contain polar and non-polar regions. The alcohol functional group (–OH) is polar due to the high electronegativity of oxygen. However, the remaining part of an alcohol consists of non-polar hydrocarbons.

 

  • The overall polarity depends on the size of the non-polar hydrocarbon group.
    • Larger alcohol molecules tend to be more non-polar because the hydrocarbon region dominates the small alcohol functional group.
    • Small alcohol molecules tend to be more polar because the polar alcohol functional group contributes more to the overall polarity of the molecule.

Ethanol is a polar molecule.

 

  • Alcohols are more soluble than hydrocarbons because they can form dipole-dipole forces and hydrogen bonds (due to the presence of hydrogen atom attached to an oxygen atom). –OH group can donate and accept hydrogen bonds to and from water molecules (shown below).

 

 

  • The solubility of alcohol in water decreases with increasing size (number of carbon atoms). For example, methanol and ethanol are very soluble in water whereas octanol is insoluble in water. Typically, hydrocarbons with more than 7 carbons are considered insoluble in water.

 

 

  • Polarity and solubility in water also differ between alcohol isomers. Isomers with shorter carbon chains tend to be more soluble because the repulsion between their non-polar regions and water is minimised.

 

For example, 2-propanol has greater solubility in water than 1-propanol; methyl-2-propanol is more soluble than 1-butanol.

 

 

Boiling and Melting Point

  • Alcohols have higher boiling and melting points than hydrocarbons with the same number of carbon atoms. This is because the presence of –OH allows the formation of dipole-dipole forces and hydrogen bonds, both of which are stronger intermolecular forces than dispersion forces.

 

Two hydrogen bonds are formed between two molecules of ethanol

 

  • Larger alcohols have higher melting and boiling points.

All alcohol molecules form dispersion, dipole-dipole and hydrogen bonds. As the size of an alcohol increases (number of electrons increases), the magnitude of dispersion force also increases. This increases their boiling and melting point.

 

  • Straight-chained alcohol molecules are able to form greater dispersion forces due to a greater surface area between molecules. As a result, straight-chain alcohols usually have higher melting and boiling point than their isomers.

 

 

Table: list of common alcohols and their boiling points. Common names are used instead of IUPAC names.

Alcohol name

Molecular formula

Molecular mass

(g mol–1)

Boiling point (ºC)

Methanol

CH3OH

32

64.7

Ethanol

C2H5OH

46

78.3

1-propanol

C3H7OH

60

97.2

2-propanol

C3H7OH

60

82.3

1-butanol

C4H9OH

74

118

2-butanol

C4H9OH

74

98.0

Methyl-1-propanol

C4H9OH

74

108

Methyl-2-propanol

C4H9OH

74

82.0