Peptide bonds, which link amino acids in proteins, are stable due to their partial double bond character. This results from the resonance or delocalisation of electrons between the nitrogen and the carbonyl carbon. Because of this resonance, the bond exhibits properties of both single and double bonds. This stability means that peptide bonds are resistant to conditions in the cellular environment and won't break down easily. This characteristic is vital for protein structure and function, ensuring that enzymes, structural proteins, and other vital cellular components retain their integrity under various conditions.

The quickest way to determine if an organic compound is saturated or unsaturated is to examine its molecular structure. If the compound contains only single bonds between carbon atoms, it is saturated. However, if there's at least one carbon-carbon double or triple bond, the compound is unsaturated. In terms of chemical tests, the bromine water test is useful. Unsaturated compounds decolourise bromine water due to addition reactions, while saturated compounds do not.

The reactivity of a functional group is determined by several factors, including its electron density, the presence of electron-withdrawing or electron-donating groups, and the overall polarity of the group. Electron-rich areas tend to be sites of nucleophilic attack, while electron-deficient regions become sites of electrophilic attack. Additionally, the strength and type of bond (e.g., single, double, or triple) within the functional group can influence its reactivity. For example, the carbon-oxygen double bond in carbonyl groups is polar and can be a site for nucleophilic addition, whereas the carbon-carbon triple bond in alkynes is less polar but more reactive due to its increased bond energy.

Common functional groups include hydroxyl (-OH) found in alcohols, carbonyl (>C=O) present in aldehydes and ketones, carboxyl (-COOH) observed in carboxylic acids, and amino (-NH₂) seen in amines and amino acids. Each of these groups imparts unique chemical properties to the molecule they are part of. For instance, compounds with a hydroxyl group often display polar behaviours due to the electronegativity of oxygen, while compounds with a carbonyl group can undergo addition reactions at the carbon-oxygen double bond.

Functional groups are specific groupings of atoms within molecules that have characteristic properties and reactivity. They play a crucial role in organic chemistry because they determine the chemical reactivity of a molecule. The presence of a particular functional group can make an organic compound behave in a certain way under specific conditions. They're like the reactive part of the molecule, enabling chemists to predict the molecule's behaviour in chemical reactions. Recognising functional groups helps in understanding the chemical and physical properties of compounds and facilitates the categorisation of organic compounds into classes or families with similar behaviours.