Investigate The Relationship Between Structure And Properties Of Oxygen Containing Organic Compounds - Session I

Alcohols are a class of organic compounds that contain a hydroxyl -OH functional group attached to a carbon atom. Do you know what the main component is of hand sanitizers? That is Alcohol. Alcohols are antiseptic and kill germs. In the structure of alcohols, the hydroxyl group is bonded to a carbon atom. The carbon atom can be part of an alkyl group. The general formula for an alcohol is ROH. R represents an alkyl group. An example of alcohol is methanol.

The alkyl group in alcohols can be simple, such as methyl or ethyl. The alkyl group can be more complex, such as propyl or butyl. In alcohols the hydroxyl group can be attached to any carbon atom in the alkyl chain. If hydroxyl group is attached to carbon atom that is further attached to another carbon atom, then alcohol is called primary alcohol. An example of primary alcohol is ethanol.

If hydroxyl group is attached to carbon atom that is further attached to two other carbon atoms directly, then alcohol is called secondary alcohol. An example of secondary alcohol is isopropyl alcohol. If hydroxyl group is attached to carbon atom that is further attached to three other carbon atoms directly, then alcohol is called tertiary alcohol. An example of tertiary alcohol is neo butyl alcohol.

Alcohol displays Polar nature due to the presence of hydroxyl group. As we know, in alcohols, hydroxyl group is attached to the carbon atom of alkyl group. Therefore, lets discuss the polar nature of carbon oxygen bond and oxygen hydrogen bond in alcohols. As we know, oxygen is more electronegative than carbon atom. The oxygen atom withdraws the electron density more towards itself. As a result, oxygen becomes partially negative. Carbon atom becomes partially positive.

Similarly, oxygen is more electronegative than the hydrogen atom attached to it. So oxygen withdraws the electron density from hydrogen towards itself. This makes oxygen partially negative. The Hydrogen atom is now partially positive. So in alcohols, the oxygen atom withdraws the electron density both from carbon and hydrogen atoms.

We already know that oxygen is more electronegative than carbon. Electronegativity of carbon is 2.5. Electronegativity of hydrogen atom is two point two. Electronegativity of oxygen is 3.4. So the electronegativity difference between carbon and oxygen atom is 0.9.

The electronegativity difference between oxygen and hydrogen atom is 1.4. This indicates that electronegativity difference between oxygen and hydrogen is greater than electronegativity difference between oxygen and carbon. Greater the electronegativity difference indicates a more polar bond. We can say that oxygen hydrogen bond is more polar than carbon oxygen bond in alcohols.

In alcohols, a hydrogen atom attached to an oxygen atom can be easily removed due to high polarity of oxygen-hydrogen bond. This indicates that alcohols can exhibit acidic properties. Acid is a specie that can easily undergo loss of hydrogen ion in solutions. For example, alcohols can dissociate in water to alkoxide ion and hydrogen ion.

Due to the presence of hydroxyl group in alcohols, they can form hydrogen bonds with other alcohol molecules. For example, ethanol molecules can form hydrogen bonds with other ethanol molecules. This type of hydrogen bonding between molecules is called intermolecular hydrogen bonding. The high boiling points of alcohols is due to hydrogen bonding between alcohol molecules.

Alcohol can also form hydrogen bonds with water molecules. This is the reason they are soluble in water. But the solubility of alcohols decreases as the length of alkyl group increases in alcohols. For example, pentanol is less soluble in water than methanol. This is because the size and length of alkyl group in pentanol is greater than methanol. This causes hurdle in making hydrogen bonds with water molecules.

We know that nucleophilic substitution of alkyl halides results in the formation of alcohols. But do you know alcohols can also undergo nucleophilic substitution to form alkyl halides? In the case of alcohols, hydroxyl group is not a good leaving group. Therefore, during nucleophilic substitution, alcohols are first protonated by an acid. This protonation results in the formation of oxonium ion. Now hydroxyl ion is converted to H2O. H2O is a good leaving group.

In the next step, water detaches as a leaving group and carbocation is formed. The carbocation is attacked by halide ion. Halide ion acts as nucleophile. Alkyl halide is formed as a product. Can you tell if this is aUnimolecular nucleophilic substitution or Bimolecular nucleophilic substitution? It is Unimolecular nucleophilic substitution because it involves the formation of carbocation.

But what will be the mechanism for bimolecular nucleophilic substitution of alcohols. Primary alcohols undergo bimolecular nucleophilic substitution. In this mechanism, protonation of primary alcohol will take place. Oxonium ion is formed. After that, in second step, halide ion attacks oxonium ion from behind. the leaving group detaches due to this attack. Primary alkyl halide is formed as a result.

Alcohols can undergo oxidation reactions to forms aldehydes or ketones. Aldehyde is formed when a primary alcohol is oxidized. For example the oxidation of ethanol results in the formation of acetaldehyde. When secondary alcohols are oxidized, ketones are formed as product. For example, the oxidation of isopropyl alcohol gives Acetone as product.

Esterification is a chemical reaction that involves the formation of an ester. Ester is an organic compound that has fruity smell. Esters have general formula RCOOR. Alcohols react with organic acids in the presence of acid catalyst to form ester. For example, the reaction of methanol with acetic acid in the presence of sulfuric acid results in the formation of methyl acetate ester. A water molecule is removed in this process. Methyl acetate has a fruity smell.