Summary Pearson Edexcel A Level Chemistry (Year 1 and Year 2) - Unit 6.2 - Hydrocarbons: alkanes and alkenes (9CH0)

6.2 - Hydrocarbons: alkanes and alkenes

Alkanes

Structure
●​ General formula: CnH2n+2 (CnH2n for cycloalkanes)
●​ ***Saturated hydrocarbons: ALL bonds are single bonds
○​ ***Hydrocarbons: molecules containing carbon and hydrogen atoms ONLY


Properties
●​ Non-polar - ACTUALLY very slightly polar​




​
∴ In fact alkanes are VERY SLIGHTLY polar (but electronegativity difference is
negligible)
●​ Insoluble in water - unable to form hydrogen bonds
●​ Less dense than water
○​ Think about it: RFM of O in H2O = 16, RFM of C in CnH2n+2 = 12 (mass
difference...)
●​ Generally unreactive/inert - due to strong C—C & C—H bonds
●​ Boiling/melting point increase with chain length - more electrons in molecules,
stronger London forces
○​ `/`` decrease with branching - number of points of contact between
molecules decreases (≠ surface area)


Reactions

Free radical halogenation
●​ ***Free radical: an atom/group with an unpaired electron (usually uncharged); highly
reactive hence short-lived (thus their ability to react with unreactive alkanes)
●​ ***Free radical substitution: substitution of hydrogen atoms on an alkane for a
halogen




1

, ●​

Mechanism***
1.​ Initiation​




2.​ Propagation​




2