haloalkanes/halogenoalkanes

primary halogenoalkane

contains a halogen atom attached to a carbon with 1 alkyl group

secondary halogenoalkane

contains a halogen atom attached to a carbon with 2 alkyl groups

tertiary halogenoalkane

contains a halogen atom attached to a carbon with 3 alkyl group

physical properties

• carbon-halogen bond is polar, haloalkanes can form PD-D attractions

• boiling points higher than alkanes and alkenes but lower than alcohols (similar molar masses)

• cannot form hydrogen bonds with water so therefore are insoluble in water

chemical properties

haloalkane contains a δ+ carbon atom and so is attacked by nucleophiles in a nucleophilic substitution reaction

nucleophile

• species that can donate a pair of electrons to a reactant in the formation of a coordination covalent bond (with the electrophile)

• can be neutral of negatively charges, must have lone pair of electrons

nucleophilic substitution reaction

• a nucleophile donates an electron pair to form a new bond as another bond breaks, producing a leaving group

• halogen atom is the leaving group for haloalkanes and is substituted, a new product is formed depending on the nucleophile

mechanism for nucleophilic substitution of haloalkane

mechanism undergone by 1° and some 2° haloalkanes

Sn2

mechanism gone by 3° and some 2° haloalkanes

Sn1

Sn2 mechanism

• nucleophilic substitution (S) involving an attacking nucleophile (n) and there are two species in the slowest step (2)

• therefore the molecularity for the slow step is 2

Sn1 mechanism

• only one species involved in the slow step, therefore the molecularity of the slow step is 1

steps of Sn2 mechanism

• first step involves the nucleophile attacking the haloalkane to form a species called a ‘transition state’

• the first step is the slow step, and it involves 2 species so the slow is bimolecular

• in the second step, the transition state breakdown to form the alcohol and halide ion (nucleophile is OH^-)

1-chloropropane + aqueous sodium hydroxide

stereospecificity of Sn2 mechanism

• Sn2 mechanism is stereospecfic, meaning that the 3 dimensional arrangement of the reactants determines the 3 dimensional arrangement of the products

• in the first step, the attakcing nucleophile attacks from the opposite side of the leaving group

• this results in the alcohol product being an inversion of the configuration at the carbon

Sn2 optical activity

• if the carbon atom is chiral and the haloalkane is 100% one optical isomer, SN2 forms 100% one optical isomer of the product.

• product is optically active

steps of Sn1 mechanism

• first step: the C–X bond breaks heterolytically, forming a carbocation and a halide ion

• the first step is the slow step and involves only 1 species, the haloalkane, so it is unimolecular

• second step: the nucleophile, OH^-, attacks the carbocation forming alcohol and X^-

2-chloro 2-methylpropane w/ hydroxide nucleophile (tertiary)

carbocation intermediate

• the carbocation intermediate in Sn1 is planar, meaning the nucleophile can attack from either above or below the plane (50:50 chance)

• two sides to attack from therefore Sn1 is not stereospecific

• a 50:50 mixture is known as a racemic/racemate mixture, and there is no optical activity

2-bromo propane w/ hydroxide nucleophile (secondary Sn1)

why 3° haloalkanes follow Sn1 mechanism

• 3° carbocations are relatively stable as they have 3 alkyl groups acting on them

• Sn2 is difficult because of steric hindrance - backside attack needed in Sn2 but tertiary haloalkane is surrounded by the three bulky alkyl groups so blocks the nucleophile

why 1° haloalkanes follow Sn2 mechanism

• if they followed Sn1, carbocation intermediate would not be stable enough (only 1 positive inductive effect)

• there is no steric hindrance to stop the nucleophile attacking the delta positive carbon

2° haloalkanes Sn2 vs. Sn1

• optically active mixture: Sn1

• not optically active: Sn2

• optically active but not 100%: mixture of Sn1 and Sn2