Alcohol’s – Basic Properties and Reactions.

 

A General Discussion.

 

            Alcohol’s are defined as a group of compounds where there is at least one –OH (hydroxyl) group bonded to an sp3-hybridised carbon.  As you can see, this is a very specific definition, designed to exclude phenols and enols, which may look similar, but have differing chemical properties.  These differences are caused by the fact that the hydroxyl group is bonded to an unsaturated organic group.  In the case of phenols, the organic (R) group is an aromatic ring, and in an enol, the –OH is attached to a double-bonded, sp2-hybridised carbon.

 

Example;

 

 

 

            Sometimes, alcohol’s are thought of as analogous to water, where one of the hydrogen’s attached to the oxygen has been replaced with an R group, as the bond geometry between the atoms in water is almost identical to that in an alcohol, if the R group is counted as one atom.

 

            Some of the most well known alcohol’s are naturally occurring.  One of the most well known alcohol’s, ethanol (C2H5OH), is produced by the process of fermentation in yeast.  This reaction has been known for centuries, and has been used in beverage making for almost as long.  Ethanol also has a place in modern society too.  It is still used in beers, wines and spirits, but it is also used on a large scale as an industrial solvent, and also as an additive to motor fuel in some of the poorer countries in the world, where there is little money oil.

 

The Naming of Alcohol’s.

 

            There are three types of alcohol – primary (1o), secondary (2o), and tertiary (3o).  This classification relates to the number of R groups bonded to the hydroxyl-bearing carbon.

 

 

 

 

Example;

 

 

 

            The IUPAC system names simple alcohol’s as derivatives of a parent alkane, using the following routine:

 

1)      Select the longest carbon chain containing the –OH group, choose the alkane of the same chain length, and replace the –e ending with –ol.

2)      Number the alkane chain beginning at the end nearest the hydroxyl group.

3)      Number the substituents according to their position on the chain, then write the name listing the substituents in alphabetical order.

 

The major properties of alcohol’s.

 

Hydrogen Bonding.

 

            As stated earlier, simple alcohol’s can be thought of as analogous to water, due to the similarity in bond geometry.  However, alcohol’s also share hydrogen bonding characteristics with water.  The major effect of hydrogen bonding is that it gives alcohol’s abnormally high boiling points compared to other organic molecules of similar structure and molecular weight.

 

Example.

 

Compound

Molecular Weight

Boiling Point (oC)

1-propanol ( C3H7OH )

60

97oC

Butane ( C4H10 )

58

-0.5oC

Chloroethane ( C2H5Cl )

65

12.5oC

 

            The reason hydrogen bonding produces these kind of results is as follows;

 

On any molecule within a liquid alcohol, the –OH group has a permanent dipole, with the oxygen carrying a small negative charge, and as a consequence, the hydrogen carries a small positive charge.  Because of these small charges, the positively polarized hydrogen of any one molecule will be attracted to the negatively polarized oxygen on another, nearby molecule.  This phenomenon occurs throughout the liquid, resulting in weak but measurable intermolecular forces extra to those which would normally occur.  These extra intermolecular forces must be overcome for a molecule to break free from the liquid phase, and enter the vapor phase.  Obviously, extra energy must therefore be supplied, and this is normally in the form of heat.  It is because of this that the boiling point is generally higher than would otherwise be expected.

 

Acidity and Basicity.

 

            Another property that alcohol’s have in common with water is the fact that they are both weakly acidic, and weakly basic too.  When acting as a base, an alcohol will be reversibly protonated by a strong acid, to yield oxonium ions, R-OH2+.

            However, when acting as an acid, an alcohol dissociate slightly in dilute aqueous solution, by donating a proton to a water molecule to give H3O+, and an alkoxide ion, RO-.

            One of the measures of acidity is the acidity constant, Ka, which is given by the following equation;

 

 

Ka  = [ A- ] [ H3O+ ]

 


      [ HA ]

 

            This equation will give rise to a huge range of numbers, and therefore they result is normally expressed as a pKa, which is simply equal to -log Ka.  Those acids with large pKa ( small Ka ) are weaker acids than those with small pKa ( large Ka ).  Here are some examples, with water and hydrochloric acid given as reference points;

 

Alcohol

pKa

(CH3)3COH

18.00

CH3CH2OH

16.00

HOH

15.74

CH3OH

15.54

CF3CH2OH

17.43

(CF3)3COH

5.4

(HCl) (not an alcohol)

-7.00

 

            Because of the fact that alcohol’s are much less acidic than carboxylic acids, or mineral acids, they do not react at all with weak bases, and only have limited reactivity with metal hydroxides.  However, they do react quite readily with alkali metals, and strong bases such as sodium hydride (NaH), alkyllithium reagents, (RLi), and Grignard reagents (RMgX).

 

Reactions of Alcohol’s.

 

            In this final section, I want to mention three reactions which alcohol’s undergo, and they are;

 

(i)                  The Dehydration of Alcohol’s to yield Alkenes.

(ii)                The Oxidation of Alcohol’s.

(iii)               The Triiodomethane ( iodoform ) reaction.

 

(i)                  The Dehydration of Alcohol’s to yield Alkenes.

 

As the title suggests, this reaction involves the breaking of the C-OH bond, along with the breaking of a neighboring C-H bond, and the formation of a C=C double bond, accompanied by the expulsion of a water molecule.

            There are numerous ways of achieving this reaction, depending upon the alcohol under consideration.  One of the most common methods used for the dehydration of tertiary alcohol’s is the acid catalyzed reaction in THF.  Let us consider a cyclic alcohol such as 1-methylcyclohexanol.  When treated with a strong acid, with THF as a solvent, then 1-methylcyclohexene is produced, along with water.

            However, this method is normally reserved for the dehydration of tertiary alcohol’s, as they are the most hardy towards these conditions, which are admittedly quite harsh.  If this reaction were to be used for either primary or secondary alcohol’s, the conditions need to be considerably more severe. For example, in the case of primary alcohol’s, the conditions would be something like 95% H2SO4, at 150oC.  Under these conditions, most molecules would just degrade.

            Because of this, other, milder conditions have been developed.  For example, it is possible to get secondary alcohol’s to dehydrate by using phosphorous oxychloride ( POCl3 ) in pyridine at 0oC, which is much less likely to degrade more sensitive molecules.

 

(ii)                The Oxidation of Alcohol’s to give Carbonyl Compounds.

 

This is one of the most common reactions of alcohol’s, with many industrial uses, as it is a relatively cheap and simple

source of aldehydes, ketones, and carboxylic acids.  However, not all three types of alcohol react.

 

Primary Alcohol’s – react to give aldehydes or carboxylic acids

 

Secondary Alcohol’s – react to give ketones

 

Tertiary Alcohol’s – DO NOT REACT

 

 

            So, as you can see, if we are to talk about alcohol oxidation, we are actually limited to two of the three types of alcohol.

            When considering primary alcohol’s, it is important to remember that they are first oxidized to an aldehyde, and the aldehyde can then be oxidized to a carboxylic acid.  With many oxidizing agents, it is also impossible to stop the reaction at the aldehyde stage, as most reagents are vigorous enough to straight through to the acid.  However, one regent which can be used is called pyridinium chlorochromate ( PCC, C5H6NCrO3Cl ) in dichloromethane.  This reagent is strong enough to oxidize from the alcohol to the aldehyde, with a respectable yield on the lab scale, without being strong enough to then oxidize the aldehyde to a carboxylic acid.

            For most other reactions, there are many oxidizing agents available for use, and the specific reagent used is decided by factors such as cost, and the sensitivity of the molecule being worked with.  Some examples of commonly used agents are KMnO4, CrO3, and Na2Cr2O7.  CrO3 in aqueous sulphuric acid is also known as Jones’ Reagent, and can be used to oxidize primary alcohol’s to carboxylic acids.

 

Example;

 

 

Jones’ Reagent

CH3(CH2)8CH2OH                                               CH3(CH2)8COOH

                                        1-Decanol                         Water, acetone                   Decanoic acid

 

            Secondary alcohol’s only give rise to one class of compound when oxidized, and these are ketones.  These ketones are normally produced in high yields.  Also, because secondary alcohol’s react quite readily, inexpensive reagents can be used to obtain these high yields. For example;

 

 

(iii)               The Triiodomethane (iodoform) Reaction.

 

The final reaction I want to mention, and only briefly, is the iodoform reaction.  It is often used to distinguish between alcohol’s containing this group;

 

and those that don’t.  Therefore, a reaction is seen with alcohol’s such as ethanol and propan-2-ol, but not with

 methanol or propan-1-ol.

            To perform the reaction, the alcohol under question is warmed with a solution of sodium hydroxide and iodine.  If the contains the above group, then a yellow precipitate of triiodomethane is formed by the following reaction;