Halagenoalkane Flashcards
what is a primary halogenoalkane
when a halogen is attached to a carbon that itself is attached to one other alkyl group
what is a secondary halogenoalkane
when a halogen is attached to a carbon that itself is attached to two other alkyl groups
what is a nucleophile
electron-rich species that can donate a pair of electrons
how are alcohols formed
aqueous sodium or potassium hydroxide, to produce alcohols in a nucleophilic substitution reaction. The hydroxide ion acts as a nucleophile. This is an example of a hydrolysis reaction
how does the shortness of the bond and Mr have an impact on the rate of reaction
Shorter bonds are stronger because the atoms are held together more tightly, making the bond harder to break. As bonds become shorter, the attraction between atoms grows stronger requiring more energy to pull them apart.
The greater the Mr of the halogen in the polar bond, the lower the bond enthalpy. A lower bond enthalpy means the bond can be broken more easily. Therefore, the rate of reaction increases for halogenoalkanes as you move down the group.
reaction with water
The water molecule is a weak nucleophile, but it will eventually substitute for the halogen
This occurs much more slowly compared to when warm aqueous sodium hydroxide is used
An alcohol is produced
CH3CH2Br + H2O → CH3CH2OH + H+ + Br-
why is hydroxide a better nucleophile
as it has a full formal negative charge whereas the oxygen atom in water only carries a partial negative charge; this causes the nucleophilic substitution reaction with water to be much slower than the aqueous alkali
how can you test it
If silver nitrate solution in ethanol is added to the solution, the silver ions will react with the halide ions as soon as they form, giving a silver halide precipitate.
how can form nitriles
react with ethanolic potassium cyanide (KCN) to form nitriles in a nucleophilic substitution reaction. The cyanide ion, CN, acts as a nucleophile. This reaction adds on a carbon atom, so it can be used in synthesis routes to increase the length of carbon chains. Forms nitrile and halide
how are amines formed
An ethanolic solution of excess ammonia (NH3 in ethanol) is heated under pressure with a primary halogenoalkane
An excess of ammonia is used because the product is more reactive than ammonia so further substitution reactions could occur
The product is a primary amine and HX
how are alkenes formed
The halogenoalkanes are heated with ethanolic sodium hydroxide causing the C-X bond to break heterolytically, forming an X- ion and leaving an alkene as an organic product. Formed alkene water and NaX
what is a nucleophilic substitution
a nucleophile attacks a carbon atom which carries a partial positive charge
An atom that has a partial negative charge is replaced by the nucleophile
Halogenoalkanes will undergo nucleophilic substitution reactions due to the polar C-X bond
how is ammonia chloride and methylamine formed
Excess ammonia is used to prevent further substitution and favour the formation of a primary amine. NH3 attaches to hydrogen on N-H bond to break it.
what can be used to measure the rate of hydrolysis of halogenoalkanes
Acidified aqueous silver nitrate
what is the method used to measure rate of hydrolysis of halogenoalkanes
Set up three test tubes in a 50 oC water bath, with a mixture of ethanol and acidified silver nitrate
Add a few drops of a chloroalkane, bromoalkane and an iodoalkane to each test tube and start a stop watch
Time how long it takes for the precipitates to form
which halogenoalkane is most and least reactive
The yellow silver iodide precipitate is the fastest nucleophilic substitution reaction
This is because the C-I bond has the lowest bond enthalpy, so it is the easiest to break
The white chloride precipitate is the slowest nucleophilic substitution reaction
This is because the C-Cl bond has the highest bond enthalpy, so it is the hardest to break and will cause the Cl- ions to form the slowest
what about silver fluoride
Silver fluoride is soluble, so a precipitate will not be formed in this reaction
what are the two ways nucleophilic substitution can occur
SN1 reactions - Tertiary halogenoalkanes
SN2 reactions - Primary halogenoalkanes
why is tertiary more reactive and faster than primary
A primary halogenoalkane has only one alkyl group attached to the carbon, so it is difficult for the halide ion to escape, hence the reaction is slowest.
what does 1 and 2 stand for
the rate of the reaction (which is determined by the slowest step of the reaction) depends on the concentration of only one reagent, the halogenoalkane
explain the Sn1 mechanism
In the first step, the C-X bond breaks heterolytically and the halogen leaves the halogenoalkane as an X- ion (this is the slow and rate-determining step)
This forms a tertiary carbocation (which is a tertiary carbon atom with a positive charge)
In the second step, the tertiary carbocation is attacked by the nucleophile
explain sn2 reactions
In primary halogenoalkanes, the carbon that is attached to the halogen is bonded to one alkyl group
The SN2 mechanism is a one-step reaction
The nucleophile donates a pair of electrons to the δ+ carbon atom of the halogenoalkane to form a new bond
the C-X bond is breaking and the halogen (X) takes both electrons in the bond
The halogen leaves the halogenoalkane as an X- ion
what would you add to dissolve precipitate
concentrated ammonia
why do tertiary react faster then secondary and primary
Tertiary is the fastest / primary is
the slowest
The C-Br bond is weakest in
2-methylbromopropane / in the
tertiary (compound)
(This is because the) methyl
groups donate electrons and tertiary are electron rich and less electronegative carbon atoms
what happens if halogalkane is reacted with OH- in aqueous conditions.
If it occurs in aqueous conditions, a nucleophilic substitution occurs, resulting in the formation of an alcohol. However, when it occurs in ethanol under reflux conditions, an alkene is produced in an elimination reaction.
rate of hydrolysis on structure of bromoalkane
Tertiary is the fastest / primary is
the slowest
The C-Br bond is weakest in
in the tertiary (compound)
(This is because the) methyl
groups donate electrons
Why is formation of alcohol better with alkali then water
Hydroxide ion /OH− is a stronger
nucleophile (than water) as full negative charge rather than partial so better electron pair donor
why is ethanol added to test tube when doing the rate of hydrolysis of halogenoalkane
to dissolve the halogenoalkane
why is apparatus left for 5mins before silver nitrate added
allow temperature to equilibrate and reach 50 degrees
