Halogenoalkanes Flashcards
Halogenoalkane
Alkane with at least 1 halogen atom in place of a hydrogen atom
Can be more than one halogen
What can halogenoalkanes with just 1 halogen atom form?
Primary, secondary or tertiary halogenoalkanes
Primary halogenoalkane
On the carbon which has a halogen attached, it has:
2 hydrogen atoms
1 alkyl group
Secondary halogenoalkane
On the carbon which has a halogen attached, it has:
1 hydrogen
2 alkyl groups
Tertiary halogenoalkane
On the carbon which has a halogenoalkane attached, it has:
3 alkyl groups
Examples of nucleophilic substitution of halogenoalkanes
Water in hydrolysis = alcohol
Aqueos KOH = alcohol
Cyanide ions = nitriles
Ammonia = amines
How can halogenoalkanes form alcohol?
In hydrolysis (add water)
What is the hydrolysis of halogenoalkanes called?
Nucleophilic substitution
Adding water to halogenoalkane mechanism
H2O splits: OH- ion substitutes halogen atom in halogenoalkane forming an alcohol, a halide ion and H+ ion
How to compare reactivities of halogenoalkanes using hydrolysis
Mix halogenoalkane with water = the halide ions substituted form which react with silver nitrate solution added to form silver halide precipitates
Time how long precipitate takes to form because this can show faster rate of hydrolysis
What determines the rate of hydrolysis of halogenoalkanes?
The carbon - specific halogen bond enthalpy
Weaker bonds break easier
What determines bond enthalpy of carbon-halogen bond?
Larger halogen radii causes longer carbon to halogen bond so a lower bond enthalpy so a weaker bond
What halogenoalkane has the fastest rate of hydrolysis ?
Iodoalkanes
If we want to speed up hydrolysis of halogenoalkane to alcohol, what can we do instead?
Use a aqueos hydroxide solution eg KOH because there are OH- ions which are stronger nucleophile
Hydroxide ion nucleophilic substitution
C d+ attracts lone pair of electrons from OH- ion
Causing heterolytic fission of C-halogen bond
So new bond = OH group bonds to carbon and halogen ion is formed
What are all the nucleophilic substitution reactions with halogenoalkanes?
+ Water/ OH- ions = alcohol
+ ammonia ion = amine
+ cyanide ions = nitrile
What are amine compounds called?
Normal naming system + amine as functional group
Product of ammonia + halogenoalkane
An ammonium ion
A halide ion
And an amine where NH2 is replacing halogen atom which has lone pair of electrons on N
Conditions for ammonia + halogenoalkane
Heated in ethanol conditions in a sealed tube (ammonia is a gas)
EXCESS AMMONIA
What happens if ammonia is limiting?
A mixture of products may form: Amine can act as nucleophile so substitutes halogen on excess haloalkanes
Because amine has lone pair of electrons so is a strong nucleophile = secondary and tertiary amines
Elimination reactions of halogenoalkane with an ethanolic alkali
Forms an Alkene by eliminating hydrogen + halogen atoms
Forms water from OH- and H+ removed from halogenoalkane
Forms salt from halogen atom+ metal in alkali
Elimination reaction products
An Alkene
Water
A salt
Conditions for elimination reaction
the alkali is dissolved in ethanol
Reflux
Halogenoalkane + aqueous alkali
Vs
Halogenoalkane + ethanolic alkali
Aqueos alkali forms an alcohol in nucleophilic substitution
Ethanolic alkali forms an Alkene in elimination reaction
Cyanide ions are?
CN-
Halogenoalkane + cyanide ions forms?
A nitrile which is the carbon chain + C≡N
+ halide ions
When adding cyanide ions to a halogenoalkane what happens to the carbon chain?
Increases by 1 because C is added from cyanide ions
Sn 1
1 reactant molecule in step 1:
The C-X bond breaks to form a carbocation intermediate
THEN IN NEXT STEP, nucleophile attacks carbocation
Intermediate in sn 1 is what shape?
Trigonal planar: carbocation is bonded to alkyl group/ hydrogens but is electron deficient because when X-C broke, electrons in bond were given to X
Intermediate in Sn 1 has what charge?
+
A carbocation
What haloalkanes undergo Sn1?
Tertiary haloalkanes
Why do tertiary haloalkanes undergo Sn1?
Because of steric hindrance where nucleophile cannot attack C d+ in the first step because alkyl groups attached to it are too large
Does Sn1 produce an optically active mixture?
No
Carbocation intermediate is trigonal planar so nucleophile can attack above or below molecular plane resulting in 2 enantiomers (racemic mixture) which rotational effect is CANCELLED OUT
Sn1 rate equation
Only haloalkane is present because its the ony molecule in rds so it must be 1/2 order, nucleophile is in second step so must be 0 order
Sn2
2 reactant molecules in step 1: nucleophile attacks C d+ and C-X bond breaks
Intermediate in Sn2 has what shape?
Trigonal bipyramidal (5 bonds) because central carbon Is bonded to 3 hydrogen/alkane groups, and dashed lines to the leaving halogen group and attacking nucleophile
SQUARE BRACKETS
Overall charge of intermediate in Sn2
-1 because carbocation +1
Nucleophile -1
Leaving halogen group -1
What haloalkanes undergo Sn2?
Primary haloalkanes
Why do primary haloalkanes undergo Sn2?
Bonded to few alkyl groups so nucleophile has enough room to attack Cd+
Does Sn2 produce an optically active mixture?
Yes (provided chiral carbon is present)
Nucleophile can only attack in direction of leaving halogen group so can only form 1 enantiomer
Which rotates plane of plane polarised light
