Quick questions on Alkanes, alkenes and alkynes explained: HSC Chemistry Module 7

A double bond is one $\sigma$ plus one $\pi$ bond; a triple bond is one $\sigma$ plus two $\pi$ bonds. The $\pi$ electrons are loosely held and are the site of attack in addition reactions.

Across all three series, increasing chain length raises both melting point and boiling point because the dispersion forces between molecules grow with molecular size. C1 to C4 hydrocarbons are gases at room temperature, C5 to about C16 are liquids, and longer chains are waxy solids. All hydrocarbons are non-polar, immiscible with water, and float on water because their density is below 1 g/mL.

Complete combustion (excess $O_2$) gives carbon dioxide and water. The general equation for any $C_xH_y$:

Alkanes are unreactive towards most reagents at room temperature. With halogens (chlorine or bromine) in UV light, they undergo free radical substitution:

Addition reactions break the weaker $\pi$ bond and add two new groups across the former double bond, leaving a saturated product.

Alkynes undergo combustion and addition like alkenes, but each $\pi$ bond can be added across in turn. So ethyne plus excess bromine gives 1,1,2,2-tetrabromoethane:

To distinguish a saturated hydrocarbon (alkane) from an unsaturated one (alkene or alkyne), add a few drops of bromine water and shake.

Alkenes add (no atoms are lost); alkanes substitute (an H is replaced and HX is a byproduct). Different mechanisms, different conditions.

Hydrogenation needs $Ni$ or $Pd$ catalyst and heat. Hydration needs dilute sulfuric acid and heat. Bromination of alkenes needs neither, only room temperature.

The H adds to the carbon with more Hs already. Think "the rich get richer" for H atoms.

Alkanes generally burn cleanly; alkenes and alkynes are sootier. If asked to compare flames, mention the C:H ratio.