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
Mechanism for hydrolysis of halogenoalkane (nucleophilic substitution) step 1
Polar bond between carbon and halogen in the halogenoalkane where carbon is d+ and halogen is d-
Mechanism for hydrolysis of halogenoalkane (nucleophilic substitution) step 2
Nucleophile (electron pair donor) is attracted to the C d+
(Could be H2O, OH- etc)
Mechanism for hydrolysis of halogenoalkane (nucleophilic substitution) step 3
The halogen is substituted for nucleophile
Because heterolytic fission of carbon-halogen bond occurs to the halogen atom forming halogen ion and nucleophile bonds to C d+
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
General equation for hydrolysis of halogenoalkane (words)
Halogenoalkane + OH- or H2O
Will form:
Halogen ion
(+ metal ion if alkali hydroxide)
Hydrogen ion
And an alcohol with remaining atoms
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
Step 1 of amine + halogenoalkane
Dipole across Cd+ and Xd-
Lone pair of electrons on nitrogen on NH3 is attracted to d+ carbon
Heterolytic fission of C-X bond
Step 2 of ammonia + halogenoalkane
Forms NH3 in place of halogen and halogen is removed to be an ion
Step 3 of ammonia + halogenoalkane
Nitrogen can only form 3 bonds but is bonded to 3 hydrogen and carbon so is electron deficient (becomes N+)
Step 4 of ammonia + halogenoalkane
Another ammonia molecule has nitrogen’ lone pair of electrons attracted to H d+ (dipole across N-H bond)
Step 5 of ammonia + halogenoalkane
Heterolytic fission of N-H bond where electrons from H goes to N
And H+ ion forms dative bond to the ammonia molecule - ammonium ion
Product of ammonia + halogenoalkane
An ammonium ion
A halogen 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
Because amine has lone pair of electrons so is a strong nucleophile
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
