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  Alkanols-Characteristics of Alkanols, Chemistry tutorial

Introduction:

We are familiar with the categorization of alkanols into primary, secondary or tertiary based on the position of the -OH group in the molecule. The sources of alkanols is although of principal of the methods used for the production of the most significant member of the series, ethanol. Ethanol is generated mostly through hydration of ethene and the fermentation of glucose.

Intermolecular Hydrogen Bonding in Alkanols:

Remember that the hydrogen bonding exist only in compounds having groups which have highly electronegative atoms, such as O, N (having lone pair of electrons) linked to hydrogen. The -OH of alkanols lead to the intermolecular hydrogen bonding among the alkanol molecules.

504_Intermolecular Hydrogen Bonding in Alkanols.jpeg

Fig: Intermolecular Hydrogen Bonding in Alkanols

H-bonding is a stronger bond than the Vander Waal's forces, which are acknowledged to hold molecules altogether. Therefore, alkanols exist as aggregates of molecules and not as individual molecules existing in the isolation.

Physical Properties of Alkanols:

1) The physical properties of alkanols exhibit the normal gradation related by any homologous series. They are, though, considerably influenced by hydrogen bonding between the alkanol molecules. This causes some level of intermolecular association in alkanols, as it does in water, so that the boiling points of alkanols are higher than those of other classes of compounds having similar molecular mass in which there is no H- bonding. For illustration:

Table:

Compound

Molecular mass

B.P. oC

Benzene (C6H6)

78

80

Pentane (C5H12)

72

36

Chloropropane (C3H7Cl)

79

47

Ethoxyethane (C4H10O)

74

35

Butanal (C4H8O)

72

76

*Butan-1-ol (C4H10O)

74

118

* Water (H2O)

18

100

Note: * hydrogen bonding exists in such molecules, therefore their high boiling point.

2) The -OH group is polar and as estimated alkanols are soluble in water. The lower members of series are extremely soluble in water; though, solubility reduces as the alkyl group or molecular mass increase in the series. This is due to the reason of the organic character (determined by the alkyl group, R) increases by molecular mass down the series whenever compared to the relative polar character (determined through the -OH group).

Chemical Properties of Alkanols:

Primary and secondary, however not tertiary alkanols can be simply oxidized and most alkanols as well undergo elimination (or dehydration) and substitution reactions. The substitution reaction can be of two types. The hydrogen atom of the hydroxyl group can be replaced by the O-H bond being split. In such reactions the alkanols act as an acid and the R-O- ion is formed as intermediate that is, ROH ↔ RO- + H+

Alternatively, the whole hydroxyl group might be replaced with the C-O bond splitting. Direct substitution of the hydroxyl groups is not simple however it is facilitated in acid solution.

Oxidation Reactions:

Primary and secondary alkanols are oxidized, through dehydrogenation, to form alkanols and alkanones correspondingly. Alkanals and alkanones are the two significant homologous series of organic compounds. The oxidation can be accomplished by using oxidizing agents like KMnO4 or K2Cr207.

The reactions comprise the loss of the -OH hydrogen altogether with the hydrogen atom from the adjacent alkyl group. By primary alkanols, the initial alkanal product might be oxidized further to an alkanoic acid, whenever surplus oxidizing agent is employed, however alkanones can't be readily oxidized.

1321_Oxidation reactions-Alkanols.jpg

Fig: oxidation reactions-Alkanols

The tertiary alkanols have no hydrogen atom on the alkyl group adjacent to the hydroxyl group and they are, thus, not readily oxidized.

Elimination (dehydration) Reactions:

Alkanols are readily dehydrated (elimination of H2O) on heating with concentrated tetraoxaolphate (IV) acid to give alkanes. The best yields of alkenes are obtained at 170oC and by employing excess concentrated H2SO4. This dehydration is just possible in molecules having at least one hydrogen atom on the carbon atom subsequent but one to the -OH group.

141_Elimination reaction.jpg

Fig: Elimination reaction

Whenever the alkanol is present in excess, though, the dehydration takes place between the two molecules to form the other class of compounds termed as alkoxy alkanes.

2R-CH2OH + H2SO4 → (140oC) → R-CH2OCH2R + H2O

 Alkanol                                         Alkoxyalkane

Substitution Reactions:

a) Reaction with sodium: The acidic nature of the -OH group shows up in their reaction by reactive metals, like sodium, to release hydrogen gas.

2R-O-H + 2Na → 2R-O-Na+ + H2

b) Esterification: The alkanols react by alkanoic acids to form alkanoates a series of compounds.

Alkanoates are generally termed as esters (therefore the word esterification for this reaction) and are generally sweet-smelling liquids. The reaction comprises the replacement of the -OH group of alkanols, therefore illustrating a fundamental character. The reactions are both slow and reversible; and are thus catalyzed by employing small amounts of strong acids example: H2SO4.

2093_Esterification.jpg

Fig: Esterification

Iodoform Test:

Ethanol and methanol react by a mixture of iodine and sodium hydroxide to form a yellow precipitate of triodomethane CH3 (Iodoform).

CH3CH2OH + 4I2 + 6NaOH → (Δ) → CHI3 + HCOONa + 5NaI + 5H2O

This reaction is employed to confirm the presence of certain kinds of alkanols namely, methanol, ethanol and other alkanols having the -OH group on a carbon atom adjacent to a methyl (-CH3) group.

Laboratory Test for Alkanols:

The presence of alkanols is confirmed mainly based on the differences in the rate of reaction of primary, secondary and tertiary alkanols by Lucas reagent (that is, mixture of concentrated HCl and Zncl2)

R-OH → (Conc. HCl and ZnCl2) → R-Cl

At room temperature, a tertiary alkanol reacts by Lucas reagent to give instantly turbidity (that is, cloudy) of the chloride. Secondary alkanols as well react at room temperature, though, the reaction is a lot slower and the 'tertiary only appears after around 5 minutes. Primary alkanols doesn't react by Lucas reagent at room temperature at all, though; the turbidity can be noticed whenever the reaction mixture is heated.

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