6D: Halogenoalkanes Flashcards
What is a halogenoalkane?
An alkane with at least one halogen atom in place of a hydrogen atom
What are the three types of halogenoalkanes?
Primary Secondary Tertiary
What is a primary halogenoalkane?
Carbon with the halogen only bound to one/no alkyl groups
What is a secondary halogenoalkane?
Carbon with the halogen is bound to two alkyl groups
What is a tertiary halogenoalkane?
Carbon with halogen is bound to three other alkyl groups
How are halogenoalkanes turned into alcohols?
Hydrolysed by nucleophilic substitution reaction Uses water
What is the general equation for the hydrolysis of halogenoalkanes to alcohols?
R-X + H2O -> R-OH + H+ + X-
How can you test the reactivity of halogenoalkanes?
Add water and silver nitrate
Water causes halide ions which reacts to form precipitate
Add both to a halogenoalkane and time how long to form a precipitate
Faster the precipitate forms, the faster the rate of hydrolysis is for that halogenoalkane
What is required to hydrolyse a halogenoalkane?
Break the carbon-hydrogen bond
What does bond enthalpy have to do with rate of hydrolysis?
The lower the bond enthalpy - the lower the bond strength Therefore it reacts faster
What halogens have the weaker C-X bond?
Iodine has the weakest bond As size of halogen is largest the C-X bond is longer and therefore weaker
What are the reactivities of primary to tertiary halogenoalkanes?
Tertiary most reactive
Why is the carbon-halogen bond polar?
Halogens are generally more electronegative than carbon
What can the carbon bonded to a halogen be attacked by and why?
Nucleophile Carbon is electron-deficient (slight positive charge)
Name some nucleophiles that attack halogenoalkanes?
OH- NH3 CN- Water is a weak nucleophile
What are the two forms of nucleophilic substitution and when do they occur?
SN1 - nucleophilic substitution 1 ( 1 species in rate determining step) Occurs with tertiary halogenoalkanes SN2- nucleophilic substitution 2 ( 2 species in rate determining step) Occurs with primary halogenoalkanes
What is the reaction to produce alcohols from halogenoalkanes?
Warm aqueous potassium hydroxide and do reaction under reflux Water not used as it is a worse nucleophile
What is the reaction between cyanide ions and halogenoalkanes?
Reflux a halogenoalkane with potassium cyanide in ethanol to form a nitrile
What is the reaction between halogenoalkane and cyanide ions?
R-X + CN- → R≡CN + X-
Triple bond between carbon in nitrile group and the carbon in R group
What is the formula for an amine?
R-NH2
What is the reaction general formula forming amines from halogenoalkanes?
R-X + NH3 -> R-NH2 + NH4+ + X-
What are the coniditions for the formation of amines from primary halogenoalkanes?
Warm a halogenoalkane with ammonia dissolved in ethanol
What do amines smell like?
Fishy smells
What happens when halogenoalkanes react with ethanolic alkali (e.g. KOH)?
Alkene, water and bromide forms
C2H5Br + KOH -> C2H4 + H2O + KBr
What are the conditions and reactants for an elimination reaction with a halogenoalkane?
Warm alkali dissolved in ethanol (e.g. KOH)
Heated under reflux
What do the hydroxide ions do in the elimination?
Hydroxide ions act a as a base to remove H+ form the halogenoalkane to form an alkene
Why is it useful to substitute a halogen with cyanide ion?
Increases the carbon chain by one
