Chapter 12 Alkanes

general formula of alkanes

C_nH_2n+2

define sigma bond

the result of the direct overlap between orbitals of two bonding atoms

why does each carbon atom have a 3d tetrahedral shape

each atom is surrounded by four electron pairs in four sigma bonds which repel

why can atoms in alkanes rotate freely

sigma bonds acting as axes

explain the trend of boiling points in alkanes

1) increases as chain length increases 2) molecules have a greater surface area so more contact is possible between molecules 3) number of London forces increases so they become stronger 4) more energy required to overcome the forces

explain effect of branching on boiling points of alkanes

1) branched alkanes have fewer surface point of contact between molecules of the branched alkanes

2) therefore there are fewer London forces

3) also the branches prevent branched molecules getting as close as straight-chain molecules

4) this decreases the London forces further

explain the low reactivity of alkanes

1) c-c and c-h sigma bonds have high bond enthalpies 2) c-c bonds are non-polar 3) c-h bond considered to be non-polar

chemical equation for complete combustion of methane

CH₄ (g) + 2O₂(g) → CO₂ (g) +2H₂O (l)

chemical equation of incomplete combustion of methane

2CH₄ (g) + 3O₂ (g) → 2CO (g) + 2H₂O (l)

general formula for balancing the complete combustion of alkanes

C_xH_y+ (x+ y/4)O₂ = xCO₂ + (y/2)H₂O

describe radical substitution with bromine

1) INITIATION: the covalent bond in a Br molecule is broken by homolytic fission, forming two Br radicals. the energy is provided by UV radiation.

2) PROPAGATION: a) a Br radical reacts with a C-H bond in the methane to form a methyl radical + HBr

b) each methyl radical reacts with another Br₂ forming bromomethane + a new Br radical. these two steps continue to cycle. propagation is terminated whenever two radicals collide.

3) TERMINATION: two radicals collide forming a molecule with all electrons paired. reactions stops when both radicals are removed from the reaction mixture.

describe further subsitution

in the second propagation step another bromine radical can collide with a bromomethane molecule, substituting a hydrogen atom to form dibromoethane CH₂Br. further substitution continues until all H atoms have be substituted

diagram of initiation of bromine

diagram of both propagation steps of bromine

diagram of termination of bromine

state two limitations of radical substitution in organic synthesis

further substitution, substitution at different points at a carbon chain

define radical

a species with an unpaired electron

why are the propagation steps considered a chain reaction

the first reacting radical is regenerated