Halogenalkanes Flashcards
What are halogenalkanes
A halogenalkane is an alkane with at least one halogen atom in place of a hydrogen atom
Describe the polarity of halogenalkanes
Halogenoalkanes are generally much more electronegative than carbon
So, most carbon-halogen bonds are polar
Why cam halogens be attacked by nucleophiles
The delta positive carbon does not have enough electrons
This means it can be attacked by a nucleophile
What is a nucleophile
A nucelophile is am electron pair donor
It donates an electron pair to somewhere without enough electrons
Wht are the conditions for the reactions of a haloagenalkane with OH-
halogenalkanes will react with hydroxide to produce alcohol
REAGENT: warm aqueous sodium or potassium hydroxide
How do we make a nitrile from halogenoalkanes
If you warm halogenoalkane with ethanolic potassium cyanide you get a nitrile
CONDITIONS: react the halogenoalkane with potassium cyanide under reflux
The ethanolic potassium cyanide dissociated to form a K+ ion and a CN- ion
Its the CN- ion that acts as the nucelophile in the reaction
How do you .are amines from halogenoalkanes
If you react warm halogenoalkne with excess ethanolic ammonia (ammonia dissolved in ethanol)
heating under pressure in a sealed tube
in the reaction, the ammonia swaps places with the halogen to form an amine
What is the reactivity if halgenoalkanes
Teaching halogen bond strength (or enthalpy) decides reactivity
For a reaction to occur the carbon -halogen binds needs to break
The C-F bond is rather strongest- it has he highest bind enthalpy
So fluroalkanes undergo nucleophillic substitution reaction more slowly than other halogenalkanes
Thr C-I bond has the lowest bond enthalpy so it its easiest to break
This means that iodoalkanes are substituted more quickly
Sum up nucleophillic subsititution
- Nucleophillic substitution reactions can occur between a galogenoalkane and a nucelophile
- The nucleophile attacks the delta positive carbon atom which breaks the carbon halogen bond
- One new bond is formed (between the nucleophile and the delta positive carbon atom) and one bind is broken (the carbon halogen bond)
4 when you’re drawing mechanisms for nucleophillic substitution reactions
, its’s important to deaw the curly arrows coming from the electrons and going to an atom. The electrons can come from either a bond or form s lone pair on an atom or ion
When does an elimination reaction occur
If you warm a halogenalkane with hydroxide ions dissolved in ethanol instead of water, an elimination equation happens and you end up with an alkene
What are the conditions for an elimination reaction
Potassium hydroxide dissolved in WARM ETHANOL under REFLUX
Nucleophilloc substitution vs elimination
You can influence which type of reaction will happen by changing the conditions
By reacting a halogenoalkane with WATER under REFLUX , the molecule will predominately undergo nucleophillkc substitution to fork an alcohol
By reactinh s halogenalkand with ethanol under reflux, the molecule will predominately undergo elimination to form an alkene
If you use a mixture kf bjtg water and ethanol as the solved, both reactions will happen and you’ll get a mixture of the two products
how do we determine the reactivity of the halogenoalkanes
test tube hydrolysis of halogenoalkanes
describe a test tube hydrolysis of a halogenoalkane
when a halogenoalkane is hydrolysed
it forms an alcohol, a halide and an h+ ion
aqueous silver nitrate is added to a halogenoalkane
The halide leaving group combines with a silver ion to form a silver halide precipitate
the precipitate only forms when the halide ion has left the halogenoalkane and so the rate of formation of the precipitate can be used to compare the reactivity of the different halogeoalkanes
what does the rate of the substitution reactions depend on
the rate of these substitution reactions depends on the strength pf the C-X bond
The weaker the bond, the easier it is to break and the faster the reaction
describe how you would carry out fractional distillation in a lab
describe why a tertiary halogenoalkane undergoes nucleophilic substitution differently
