ALCOHOL
Introduction It is an organic compound in which a hydroxyl group (–OH) is attached to an alkyl group. General molecular formula of alcohol is CnH2n+1OH and structurally it can be represented as
The hydroxyl group is functional group and all chemical properties of alcohol depend on it. Nomenclature and Structures Alcohols are named based on alkyl group to which hydroxyl group is attached. Some of the alcohols with their common name, IUPAC name and their structure are listed below
Alcohols are classified as 10, 20, or 30 depending upon the type of carbon to which hydroxyl group is attached. Alcohols are also named as monohydric, dihydric or trihydric depending upon the number hydroxyl groups attached to the alkyl group.
Physical properties Lower alcohols such as methanol ethanol etc are colorless toxic liquids with characteristic smell. The boiling points of the alcohols increase as the number of carbon atoms increases. The patterns in boiling point reflect the patterns of intermolecular forces and it is same as alkanes. The boiling point increases with the increase in molecular mass. The boiling point decreases with increase in branching. The boiling point of an alcohol is always much higher than that of the alkane with the same number of carbon atoms. This is due to hydrogen bonding. Alcohols are more soluble in water in comparison to alkane. This is again due to hydrogen bonding. Other wise solubility also shows the same trend as shown by alkane. Hydrogen bonding occurs between molecules where there is a hydrogen atom attached to one of the very electronegative elements - fluorine, oxygen or nitrogen. In case of alcohols, hydrogen bonds are set up between the slightly positive hydrogen atoms and lone pairs on oxygen in other molecules. Synthesis of Alcohols Alcohol can be prepared by various methods. Any one of the methods listed below can be used depending on the type and yield of alcohols required.
Reduction of carbonyl group: Carbonyl group is present in aldehydes, ketones and carboxylic acids. All these compounds can be reduced by either using direct hydrogen in presence of a catalyst or by using a reducing agent.
Both NaBH4 and LiAlH4 can be used to reduce aldehyde and ketones. Reduction of aldehydes will give primary alcohols while reduction of ketones will give secondary alcohols. In all the four reaction shown above hydride ion acts as a nucleophile.
Reducing agent attacks carbonyl carbon, breaks the double bond on oxygen, and forms alcohol. The hydride ion preference to carbonyl carbon is based on the electron donating tendency of the group attached to carbonyl carbon. More easily the electron is donated less likely the hydride ion gets attacked and forms alcohol. The tendency to form alcohol can be graded as
Reduction using Grignard Reagents(R – MgX) Grignard reagent is an organometallic compound having general formula as R – MgX (X = F,Cl, Br, I) These are highly polarized species due to polar bond between the metal and carbon. Carbon is more electronegative than the magnesium hence carbon becomes partially negative. The polarized carbon metal bond and a partially negative charge on the carbon make it a strong nucleophile and a base. Grignard reagent also reacts with C = N, S = O, N = O. It
deprotonates O – H, N – H, S – H. Grignmard reagent is made in ether. Carbon of carbonyl is positively charged as oxygen of carbonyl group is more electronegative than carbon. Thus it acts as an electrophile. The incoming nucleophile carbon of Grignard reagent attacks the carbonyl group carbon.
While adding to carbonyl group, organic part of Grignard reagent attaches itself to carbon and magnesium to oxygen of carbonyl group. The product is a weak salt of acidic alcohol which is easily converted to alcohol by giving it an acidic bath. Primary alcohol is produced by the reaction between formaldehyde and Grignard reagent.
Secondary alcohol is obtained by treating Grignard reagent with all other aldehydes other than formaldehyde.
Tertiary alcohol is produced due to the reaction between Grignard reagent and ketones.
In all the above reactions carbon chain is extended by one carbon. Grignard reagent therefore can be used to synthesize organic compounds i