Alkane properties

What are alkanes, and what makes them saturated?

Alkanes are hydrocarbons (containing only carbon and hydrogen) in which every carbon atom is bonded to the maximum number of other atoms through single bonds only. They are described as saturated because they contain only single C–C bonds — there are no double or triple bonds.

What type of bond are all C–C and C–H bonds in alkanes, and how does a σ bond form?

All bonds in alkanes are σ (sigma) bonds. A σ bond forms from the direct overlap of orbitals along the internuclear axis — that is, directly between the two bonding atoms. The electron density in a σ bond is concentrated along the line between the two nuclei.

Why do alkanes have free rotation around C–C bonds?

Because the only bond between adjacent carbons is a σ bond, and a σ bond is symmetrical around the internuclear axis. Rotation around this axis does not disrupt the electron overlap — the bond remains intact no matter what angle the two halves of the molecule are at relative to each other. This is in contrast to a C=C double bond, where the π bond prevents rotation.

What is the shape around each carbon atom in an alkane, and what is the bond angle?

Each carbon atom in an alkane is tetrahedral with bond angles of 109.5°. Each carbon has four bonding pairs of electrons and no lone pairs. four electron pairs repel each other and arrange themselves as far apart as possible in 3D space

What type of intermolecular force acts between alkane molecules, and why?

Alkanes are non-polar molecules because C–H and C–C bonds have very similar electronegativities, making the bonds essentially non-polar. As a result, the only intermolecular forces present are London (induced dipole–dipole) forces. These arise from temporary, instantaneous dipoles caused by uneven electron distribution.

How does chain length affect the boiling points of alkanes?

Boiling point increases with increasing chain length (number of carbon atoms). Longer chains have more electrons, so more surface points of contact due to its large surface area. This leads to stronger London forces between molecules. More energy is therefore needed to separate the molecules, so the boiling point is higher.

How does branching affect the boiling point of an alkane compared to its straight-chain isomer?

Branched alkanes have lower boiling points than their straight-chain isomers with the same molecular formula. Branching makes the molecule more compact and roughly spherical, reducing the surface area available for contact between neighbouring molecules. reduced contact surface area means weaker intermolecular forces and a lower boiling point. For example, methylpropane (branched C₄H₁₀) has a lower boiling point than butane (straight-chain C₄H₁₀).