Organics - Halogenoalkanes Flashcards
Nucleophile
A species attracted to an area of positive charge; it is an electron pair donator. (Generally most things with a lone pair is a nucleophile unless it is a positive ion with a lone pair because then it wouldn’t be attracted to a positive are, it would repel it)
Reaction of halogenoalkanes and aqueous potassium/sodium hydroxide produces alcohols [Sn1 mechanism version]
THIS IS EXAMPLE IS WITH TERTIARY
For example, (CH3)3CBr reacting with potassium hydroxide produces ethanol (it also produces potassium bromide however if I give a full equation it confuses which reagents are actually part of the mechanism).
(CH3)3CBr is mixed with water, reacted with potassium hydroxide and
- heated under reflux.
- (CH3)3CBr and all halogenoalkanes are insoluble in water which is why a 50/50 mix of water and ethanol is used*
H
I
H H-C -H
I I
H-C——C——Br
I |
H H-C-H
I
H
SLOW STEP:
The bromine is more electronegative than the carbon therefore it pulls the electrons they are both sharing further towards itself, distorting the electron cloud and weakening the bond until eventually it breaks. A free bromide ion is released.
The carbon becomes positive because it loses the electrons that it had in the bond and a carbocation forms. Btw this is heterolytic fission. (Curly arrow from C-Br bond to the Br)
FAST STEP:
The aqueous potassium hydroxide means that there are free OH- ions floating around. This is attracted to the positive carbon, it rapidly attacks the carbocation, taking the place of where the bromine originally was to form an alcohol.
H
I
H H-C -H
I I
H-C——C——OH
I |
H H-C-H
I
H
- nucleophile substitution reaction
- OH- ion is the nucleophile
The position of the Br and the OH basically swap places which it’s a substitution reaction.
Reaction of halogenoalkanes with aqueous silver nitrate in ethanol to produce a silver halide (and another product I don’t think we need to know what this is but look it up anyway!!!!!)
For example, silver nitrate (AgNO3) mixed in ethanol reacting with methyl chloride (CH3Cl) produces silver chloride (AgCl) and also methyl nitrate (CH3 NO3) but I don’t really need to know the last product.
JUST KNOW: halogenoalkane + silver nitrate in ethanol —> silver halide + {}
Mechanism:
Chlorine is much more electronegative than carbon therefore it pulls the electrons both atoms share in the C-Cl bond towards itself, distorting the cloud and weakening the bond until eventually it breaks.
The carbon becomes partially positive because it has lost the electrons it had in the C-Cl bond (basically a carbocation forms).
Aqueous silver nitrate is dissolved in water, the water with its lone pair of electrons is the nucleophile. Therefore the water is attracted to the positive carbon (basically the carbocation).
HOWEVER, in this case there is competition of the negative nitrate ion from the silver nitrate also being attracted to the carbocation.
In this case, the nitrate ion wins and it takes the place where the Cl was and forms methyl nitrate.
I don’t need to know this mechanism in detail but I’m looking at it anyway - see the diagrams in my notes
FACTSS: The reaction is SLOW because water and halogenoalkanes are immiscible therefore there is a low collision rate. Ethanol is added to allow them to mix which increases the reaction rate.
Reactions of halogenoalkanes with potassium cyanide to produce nitriles - 4 things
- potassium/sodium cyanide mixed in ethanol (important no water present otherwise an OH group substitutes the halogen rather than a CN)
- HEAT UNDER REFLUX
- means of increasing the carbon chain length
- halogen in halogenoalkane replaced by the CN
Reactions of halogenoalkanes with ammonia to produce primary amines - CONDITIONS AND REAGENTS
- concentrated solution of ethanolic ammonia
- heated in a sealed tube at high pressures
- cannot heat under refluc because ammonia would escape up condenser as a gas
- use excess ammonia to make the primary amine the major product
Reactions of halogenoalkanes with ethanolic potassium hydroxide to produce alkenes
how many arrows??
Same as reaction producing alcohols except ETHANOLIC KOH/NaOH used.
- heat under reflux
- hydroxide ion acts as a base (in the aqueous version it acts as a nucleophile)
the one with three arrows!! (from the OH to the H to the C-C to the halogen!)
Experiments and reactivities of halogenoalkanes
The rate of reaction increases down the group because less energy is required to break the carbon-halogen bond (another way of saying this is that there is a SMALLER BOND ENTHALPY). So the faster reacting halogenoalkanes are iodoalkanes because they have the weakest bond and so require less energy to break it so the reaction happens quicker.
Where > means faster reactivity:
Iodoalkanes>bromoalkanes>chloroalkanes
This can be demonstrated in an experiment:
Silver nitrate and ethanol is added to three separate test tubes containing chlorobutane (no precipitate forms because carbon-halogen bond strong so takes a very long time for it to be broken another way of saying this is that it has a HIGHER BOND ENTHALPY), bromobutane (precipitate forms after about 15 minutes) and iodobutane (precipitate forms after about 5 minutes because the carbon-halogen bond is very weak and so requires little energy to break so breaks quickly.
Overall:
- carbon-halogen bond enthalpy decreases down the group
- carbon-halogen bond becomes weaker down group
- faster reactivity down the group
Reactivities of primary, secondary and tertiary halogenoalkanes
So where > means more reactive:
Tertiary halogenoalkanes>secondary halogenoalkanes>primary halogenoalkanes
Tertiary halogenoalkanes are less stable, but they form more stable tertiary carbocations due to the positive inductive effect of the three alkyl groups. As a result they under go nucleophilic substitution more readily
Reactions of halogenoalkanes with potassium cyanide to produce nitriles - MECHANISM
Primary halogenoalkanes:
imagine bromoethane
imagine a CYANIDE ion (CN)
Bromine more electronegative than carbon so produces dipole in the C-Br bond.
Lone pair of electrons on the C in CN attract to delta positive carbon & simultaneously bromine breaks off
end up with a nitrile and a bromide ion
[this part of the transition state looks like this:
N=C ——-C———-Br
CN triple bond, C attached to the ‘bond in the making’
Full equation:
CH3CH2Br + KCN –> CH3CH2CN + KBr
Ionic equation:
CH3CH2Br + CN- —-> CH3CH2CN + Br-
Reactions of halogenoalkanes with ammonia to produce primary amines - MECHANISM
-SN2 (Primary halogenoalkane to produce a primary amine)
imagine bromoethane
imagine an NH3- (delta negative on nitrogen)
SLOW STEP:
bromine electronegative so makes the carbon in the C-Br bond delta positive. The lone pair of electrons on the NH3 attracts to this Carbon & Br simultaneously breaks off.
The nitrogen now has a + charge because it formed a bond with the carbon so effectively lost an electron.
FAST STEP:
another NH3 molecule comes along and removes a hydrogen from the NH3 attached to the ethane. The N-H bond breaks so the nitrogen is no longer sharing an electron and is effectively up an electron so it loses its positive charge.
A primary amine and ammonium form.
This is a REVERSIBLE reaction
the ammonium ion and bromine ion then combine and form an ionic bond to create ammonium bromide.
Which halogenoalkane reactions are heated under reflux?
- aqueous potassium/sodium hydroxide
- ethanolic potassium/sodium hydroxide
- potassium cyanide
