Describe what is meant by each of the following reaction types, and illustrate with an example: (a) nucleophilic substitution reaction; (b) electrophilic substitution reaction; (c) addition reaction; (d) elimination reaction; (e) rearrangement reaction.

A nucleophilic substitution reaction is a fundamental type of reaction in organic chemistry where a nucleophile, an atom or molecule with a pair of electrons to donate, replaces a group in another molecule, known as the leaving group. The nucleophile is attracted to an electrophilic center, which is an electron-deficient carbon atom. One of the simplest examples is the hydrolysis of tert-butyl bromide, where water serves as the nucleophile and the bromine atom is the leaving group.

In this reaction, \( R-Br + H_2O \rightarrow R-OH + HBr \), the \( R-Br \) compound undergoes a chemical change. The water molecule donates an electron pair to the carbon atom bound to the bromine, resulting in the formation of an alcohol (\( R-OH \) and hydrobromic acid (\( HBr \) as byproducts. Nucleophilic substitution reactions are important in the synthesis of various compounds in pharmaceutical and material sciences.

Electrophilic substitution reactions are characterized by the replacement of an atom in an aromatic compound with an electrophile, which is an electron-seeking species that can accept an electron pair. This type of reaction is common among aromatic compounds such as benzene. A classic example is the nitration of benzene, where a nitro group (\( -NO_2 \) replaces one of the hydrogen atoms on the benzene ring.

During this process, the compound benzene (\( C_6H_6 \)) reacts with nitric acid (\( HNO_3 \)) in the presence of sulfuric acid, which acts as a catalyst, resulting in nitrobenzene (\( C_6H_5NO_2 \) and water as products:\( C_6H_6 + HNO_3 \rightarrow C_6H_5NO_2 + H_2O \). Electrophilic substitution is essential for adding functional groups to aromatic rings, which is a critical step in the production of dyes, pharmaceuticals, and explosives.

Addition reactions are typical in alkenes and alkynes, where a double or triple bond opens up to allow for the attachment of other atoms or groups, resulting in a single product. A textbook example is the addition of hydrogen bromide to propene:

\( CH_3-CH=CH_2 + HBr \rightarrow CH_3-CHBr-CH_3 \).

The double bond between the carbons in propene is highly reactive, allowing for the bromine atom from the hydrogen bromide to attach to one of the carbon atoms while the hydrogen attaches to the other. This results in the compound 2-bromopropane. Addition reactions are central to creating a wide array of compounds, such as pharmaceuticals and polymers, by altering the structure of unsaturated carbon chains.

Elimination reactions are the opposite of addition reactions. They involve removing two atoms or groups from a molecule and forming a new double or triple bond between the atoms that were originally connected to the removed atoms. In the dehydration of alcohol, a common example, alcohol (such as ethanol) loses a water molecule to form an alkene:

\( C_2H_5OH \rightarrow C_2H_4 + H_2O \).

In this scenario, a hydrogen atom and a hydroxyl group (\( -OH \) leave the alcohol molecule, resulting in the formation of ethene (\( C_2H_4 \) and water. Elimination reactions play an important role in organic synthesis, particularly in the formation of alkenes from alcohols and halides.

Rearrangement reactions involve the structural reorganization of a molecule without the addition or removal of any atoms. The molecule undergoes a transformation into a different isomer. A well-known example is the rearrangement of carbocations, which often occurs during the mechanisms of other reactions such as electrophilic addition:

For a simple secondary carbocation (e.g. \( CH_3-CH^{+}-CH_3 \)), a hydrogen atom can shift from a neighboring carbon to the positively charged carbon, resulting in a more stable tertiary carbocation. These kinds of internal rearrangements are crucial for understanding many reaction mechanisms and for optimizing reaction conditions in synthetic chemistry, as they can significantly influence the outcome and yield of the reactions.