2.6 - Halogenoalkanes Flashcards
What is a halogenoalkane?
A homologous series where one or more hydrogen atoms in an alkane have been replaced by a halogen atom
In chloromethane the more electronegative chlorine atom makes the molecule δ_ and Carbon that is attached to the Cl is δ_
This means that halogenoalkanes are susceptible to attack by electron-rich species via the electropositive _____ atom. This is known as nucleophilic attack (leads to substitution, where and atom is subbed for another atom or species)
negative
positive
carbon
What happens in the nucleophilic substitution of 1-chlrobutane when in the presence of NaOH and aqueous ethanol?
The nucleophile is the hydroxide ion, OH- which is provided by NaOH. It has lone pairs on the O atom of OH, so is therefore a good nucleophile.
The lone pair of the HO ion is donated to the δ+ carbon in the chlorobutane, following attack by the nucleophile. Because the lone pair is donated, a bond is formed between the OH- and the carbon. The carbon chlorine bond breaks to release the chloride ion. The product is butan-1-ol
See NC 1
What is a nucleophile?
A species with a lone pair of electrons that can be donated to an electron deficient species
What is the general formula of a halogenoalkane?
C(n)H(2n+1)X
X is a halogen
Changing the halogen will effect the rate of reaction.(NB, only Cl, Br and I are considered) This is due to what 2 factors and why? Which halogen is hydrolysed the fastest?
Electronegativity - Cl is the most electronegative , which makes the C-Cl bond the most polar out of the 3 possible. The carbon in the C-Cl bond is the most δ+ out of the 3
Bond strength - Cl is the smallest halogen, so that the C-Cl bond is the strongest and it is this bond that is broken during hydrolysis
The iodo-compound is hydrolysed most quickly, it must be that the bond strength has the most effect
How do you test for the presence of halogenoalkanes?
Organic compound is heated with aqueous sodium hydroxide
RX + NaOH —-> ROH + Na+ + X-
R is alkyl group
X is halogen
Once X- has been produced by hydrolysis, HNO3 is added in order to remove any NaOH remaining. Then AgNO3 can be added to the test for the type of halide present
X- + Ag+ —-> AgX
What precipitate is observed when silver nitrate is mixed with a halogen (Cl, Br and I)? Do they dissolve when ammonia is then added to precipitate?
Cl - white (dissolves in dilute ammonia solution)
Br - cream (dissolves in concentrated ammonia solution)
I - yellow (doesn’t dissolve in ammonia solution)
What is an elimination reaction?
A reaction that involves the loss of a small molecule in order to produce a double bond
What happens in an elimination reaction?
A hydrogen halide is removed and an alkene is formed. The hydrogen halide is acidic (e.g. HCl) and can be removed using an alkali such as NaOH. To avoid nucleophilic substitution occurring instead of elimination, the alkali must be in ethanolic solution with water (anhydrous ethanol)
See NC 1
Nucleophilic substitution can be used to prepare alcohols from halogenoalkanes, e.g. butan-1-ol from 1-bromobutane. The nucleophile OH- is provided by aqueous sodium hydroxide. What is this reaction also classified as?
Hydrolysis (reaction with water to produce new product) since the reaction can also be achieved using water (which contains small amounts of OH-)
Since nucleophilic substitution reactions are rather slow it is necessary to heat the mixture of 1 -bromobutane and aqueous sodium hydroxide for a significant amount of time. If this were done in an open flask or beaker much of the liquids would evaporate and be lost and the yield of product would be very low. To avoid this problem the liquid is…
refluxed - a process of continuous evaporation and condensation
See NC 2
To convert halogenoalkanes to alcohols, aqueous ______ _______ and aqueous _____ and _____ are used. This substitution reaction involves a ________. (Nucleophilic substitution mechanism)
sodium hydroxide
ethanol
heat
nucleophile
To covert halogenoalkanes to alkenes, ______ ______ in ______ _____ and _____ are used. This time the mechanism is elimination and the hydroxide ion behaves as a base rather than a nucleophile.
sodium hydroxide
anhydrous ethanol (pure ethanol)
heat
Why is an elimination reaction different to a nucleophilic substitution reaction?
Because no water is present so the (e.g. NaOH) is not a nucleophile and acts a base (proton acceptor)
