Chemistry » Organic Chemistry » Alcohols and Ethers

Alcohols

In this topic, we will learn about alcohols and ethers.

Alcohols

Incorporation of an oxygen atom into carbon- and hydrogen-containing molecules leads to new functional groups and new families of compounds. When the oxygen atom is attached by single bonds, the molecule is either an alcohol or ether.

Alcohols are derivatives of hydrocarbons in which an –OH group has replaced a hydrogen atom. Although all alcohols have one or more hydroxyl (–OH) functional groups, they do not behave like bases such as NaOH and KOH. NaOH and KOH are ionic compounds that contain OH ions. Alcohols are covalent molecules; the –OH group in an alcohol molecule is attached to a carbon atom by a covalent bond.

Ethanol, CH3CH2OH, also called ethyl alcohol, is a particularly important alcohol for human use. Ethanol is the alcohol produced by some species of yeast that is found in wine, beer, and distilled drinks. It has long been prepared by humans harnessing the metabolic efforts of yeasts in fermenting various sugars:

This figure shows the reaction of glucose to produce ethanol and C O subscript 2. The reaction shows C subscript 6 H subscript 12 O subscript 6 ( a q ) arrow labeled “yeast” 2 C subscript 2 H subscript 5 O H (a q) plus 2 C O subscript 2 ( g ). The O H in ethanol is shown in red.

Large quantities of ethanol are synthesized from the addition reaction of water with ethylene using an acid as a catalyst:

This reaction shows two carbons connected by a double bond, each with two bonded H atoms plus H O H arrow labeled “H subscript 3 O superscript plus” followed by two carbon atoms connected with a single bond with 5 bonded H atoms and an O H group shown in red at the right end of the molecule. The O of this group is shown with 2 pairs of electron dots.

Alcohols containing two or more hydroxyl groups can be made. Examples include 1,2-ethanediol (ethylene glycol, used in antifreeze) and 1,2,3-propanetriol (glycerine, used as a solvent for cosmetics and medicines):

Structural formulas for 1 comma 2 dash ethanediol and 1 comma 2 comma 3 dash propanetriol are shown. The first structure has a two C atom hydrocarbon chain with an O H group attached to each carbon. The O H groups are shown in red an each O atom has two sets of electron dots. Each C atom also has two H atoms bonded to it. The second structure shows a three C atom hydrocarbon chain with an O H group bonded to each carbon. The O H groups are shown in red, and each O atom has two sets of electron dots. The first C atom has two H atoms bonded to it. The second C atom has one H atom bonded to it. The third C atom has two H atoms bonded to it.

Naming Alcohols

The name of an alcohol comes from the hydrocarbon from which it was derived. The final -e in the name of the hydrocarbon is replaced by -ol, and the carbon atom to which the –OH group is bonded is indicated by a number placed before the name.1

Example

Naming Alcohols

Consider the following example. How should it be named?

A molecular structure of a hydrocarbon chain with a length of five C atoms is shown. The first C atom (from left to right) is bonded to three H atoms. The second C atom is bonded on one H atom and an O atom which is also bonded to an H atom. The O atom has two sets of electron dots. The third C atom is bonded to two H atoms. The fourth C atom is bonded to two H atoms. The fifth C atom is bonded to three H atoms. All bonds shown are single.

Solution

The carbon chain contains five carbon atoms. If the hydroxyl group was not present, we would have named this molecule pentane. To address the fact that the hydroxyl group is present, we change the ending of the name to -ol. In this case, since the –OH is attached to carbon 2 in the chain, we would name this molecule 2-pentanol.

Extra Notes: Using Lucas’ Reagent to Classify Alcohols

Lucas’ reagent is a solution of anhydrous zinc chloride in concentrated hydrochloric acid. This solution is used to classify alcohols of low molecular weight. The reaction is a substitution in which the chloride replaces a hydroxyl group. A positive test is indicated by a change from clear and colourless to turbid, signalling formation of a chloroalkane.

Also, the best results for this test are observed in tertiary alcohols, as they form the respective alkyl halides fastest due to higher stability of the intermediate tertiary carbocation. The test was reported in 1930 and became a standard method in qualitative organic chemistry. The test has since become somewhat obsolete with the availability of various spectroscopic and chromatographic methods of analysis. It was named after Howard Lucas (1885–1963).

Lucas test: negative (left) with ethanol and positive with t-butanol. Credit: Wikimedia Commons, CC BY-SA 3.0

Lucas test in alcohols is a test to differentiate between primary, secondary, and tertiary alcohols. It is based on the difference in reactivity of the three classes of alcohols with hydrogen halides via a substitution reaction.

ROH + HCl → RCl + H2O

The differing reactivity reflects the differing ease of formation of the corresponding carbocations. Tertiary carbocations are far more stable than secondary carbocations, and primary carbocations are the least stable.

[Attributions and Licenses]


This is a lesson from the tutorial, Organic Chemistry and you are encouraged to log in or register, so that you can track your progress.

Log In

Share Thoughts