Question

(a) What is the difference between a straight-chain and branched-chain alkane? (b) What is the difference between an alkane and an alkyl group? (c) Why are alkanes said to be saturated? (d) Give an example of an unsaturated molecule.

Short answer

(a) Straight-chain alkanes have all carbon atoms connected in a straight line, while branched-chain alkanes have side chains or branches. (b) An alkane is a hydrocarbon with single covalent bonds between carbon atoms (CnH2n+2), while an alkyl group is a part of an alkane with one hydrogen atom missing (CnH2n+1). (c) Alkanes are saturated because they contain only single covalent bonds between carbon atoms and are bonded to the maximum number of hydrogen atoms. (d) An example of an unsaturated molecule is ethene (\(C_2H_4\)), which contains a carbon-carbon double bond.

Step-by-step solution

(a) Straight-chain vs Branched-chain Alkanes

Straight-chain alkanes have all the carbon atoms connected in a straight line without any branches, while branched-chain alkanes have carbon atoms connected in a way that creates side chains or branches.

(b) Alkane vs Alkyl Group

An alkane is a hydrocarbon with only single covalent bonds between carbon atoms, and each carbon atom is bonded to the maximum possible number of hydrogen atoms (CnH2n+2). An alkyl group is a part of an alkane that has one hydrogen atom missing. It can be described by the formula CnH2n+1 and can be attached to other molecules.

(c) Alkanes as Saturated Compounds

Alkanes are said to be saturated because they only contain single covalent bonds between the carbon atoms. This means that each carbon atom is bonded to the maximum possible number of hydrogen atoms, filling all available bonding sites. There are no double or triple bonds in alkanes.

(d) Example of an Unsaturated Molecule

An example of an unsaturated molecule is ethene (\(C_2H_4\)). Ethene has a carbon-carbon double bond, which makes it unsaturated. It belongs to the class of hydrocarbons known as alkenes, which contain at least one carbon-carbon double bond in their structure.

Key concepts

Straight-Chain vs Branched-Chain Alkanes

When examining the structure of alkanes, there are two main types to consider: straight-chain alkanes and branched-chain alkanes. Imagine straight-chain alkanes as a simple necklace, with each carbon atom representing a bead aligned in a consecutive sequence. There are no additional pendants or decorations – just a straight line of carbon atoms bonded to hydrogens, with a chemical formula resembling \(C_nH_{2n+2}\).

On the other hand, branched-chain alkanes are like necklaces with several pendants. They have the same carbon 'beads,' but some of these beads sprout 'branches' – that is, chains of carbon atoms that jut out from the main line. The presence of these branches can significantly alter the physical and chemical properties of the molecule, such as boiling point and reactivity.

Significance of Branching

Branching affects how molecules interact with each other. In general, branching lowers the boiling point of an alkane, as the irregular shape makes it harder for the molecules to pack closely together and thus, reduces the van der Waals forces between them.

Alkane vs Alkyl Group

Alkanes are the simplest family of hydrocarbons where every carbon atom is singly bonded to its neighbors and to hydrogen atoms, filling all available bonds. The general formula of alkanes is \(C_nH_{2n+2}\) where 'n' is the number of carbon atoms in the molecule. They are the chemical equivalent of a fully saturated sponge that cannot hold any more water; there is no room for more hydrogen atoms without breaking the existing bonds.

In contrast, an alkyl group is like an alkane with one hydrogen atom plucked away. This missing hydrogen creates a 'hook' that allows the alkyl group to link with other atoms or molecules. Expressed with the formula \(C_nH_{2n+1}\), alkyl groups are not stable molecules on their own, but they serve as important building blocks in larger organic molecules.

Understanding Alkyl Groups

Think of an alkyl group as a 'team player' in chemistry. It's not complete by itself, but when it collaborates with the rest of the molecule, it helps to construct a variety of complex structures.

Saturated Hydrocarbons

Alkanes are often referred to as saturated hydrocarbons, drawing a comparison to a sponge that is full of water and can't absorb any more. Similarly, alkanes have maximized their hydrogen capacity, binding as many hydrogen atoms as possible to each carbon atom. This maximization is due to single covalent bonds filling all possible bonding sites on the carbon atoms.

The term 'saturated' comes from the idea that the carbon chain cannot fit more atoms into its structure without altering the carbon chain. This saturation leaves the molecule very stable and less reactive when compared to 'unsaturated' counterparts, which contain double or triple bonds and can add more atoms by opening these bonds.

Implications for Reactivity

Being saturated makes alkanes relatively chemically inert substances. They do not easily participate in addition reactions that are typical for unsaturated hydrocarbons. This property is why saturated hydrocarbons are frequently used as fuels and in a variety of non-reactive contexts.

Unsaturated Molecules

While saturated hydrocarbons are 'full' in terms of hydrogen atoms, unsaturated molecules still have room for more. Unsaturated hydrocarbons such as alkenes and alkynes are characterized by their double or triple carbon-carbon bonds respectively. Ethene (\(C_2H_4\)) is a prime example of an unsaturated molecule – it includes a carbon-carbon double bond that constitutes a point of unsaturation.

Unsaturated hydrocarbons are much more reactive than saturated ones. The presence of double or triple bonds offers a site for reactions like bonding with additional atoms (hydrogenation) or molecules. This reactivity makes them valuable in chemical synthesis and as starting materials for making a vast array of chemicals.

Utilizing Double Bonds

The double bonds in unsaturated molecules are not just points of weakness — they're opportunities. These molecules are the foundation of many important chemical processes, including the production of plastics, pharmaceuticals, and other synthetic materials.