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
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
