Halogenoalkanes Flashcards
What is the general formula?
Cn H2n+1 X, where X is a halogen atom: one of F, Cl, Br or I.
How are halogenalkanes named?
By prefixing the name of an alkane with flouro, chloro, bromo or iodo and a number to indicate the position of the halogen on the hydrocarbon chain.
E.g CH3CH2CHCLCH3 is 2-chlorobutane
Name this compound- CH3CH2CH2I
1-Iodopropane
Name this compound- CH3CHBRCH3
2-bromopropane
Name this compound- CBrF2CBrF2
1,2- dibromo- 1,1,2,2- tetraflouroethane.
How are halogenalkanes classifiied?
According to their structures.
What is a primary halogenoalkane?
E.g 1- chlorobutane, the halogen atom is covalently bonded to a carbon atom which, in turn, has a covalent bond to just one other carbon atom.
What is a secondary halogenoalkane?
E.g- 2 chlorobutane, the halogen atom is covalently bonded to a carbon atom which, in turn has covalent bonds to two other carbon atoms
What is a tertiary halogenoalkane?
E.g 2-chloro- 2- methyl propane, the halogen atom is covalently bonded to a carbon atom which, in turn, as covalent bonds to three other carbon atoms.
What are the physical properties of halogenoalkanes?
Typically they are volatile liquids that do not mix with water.
What is the polarity of the carbon-halogen bonds?
With the exception of iodine, all of the halogens are more electronegative than carbon.
That means that the electron pair in the carbon-halogen bond will be dragged towards the halogen end, leaving the halogen slightly negative (-) and the carbon slightly positive (+) - except in the carbon-iodine case.
What is the trend in bond strength?
In halogenoalkanes, the bonds get weaker as you go from chlorine to bromine to iodine.
What is a nucleophile?
A nucleophile is a species (an ion or a molecule) which is strongly attracted to a region of positive charge in something else.
Nucleophiles are either fully negative ions, or else have a strongly - charge somewhere on a molecule. Common nucleophiles are hydroxide ions, cyanide ions, water and ammonia.
Explain the nucleophilic substitution reaction, using bromoethane as a typical primary halogenoalkane and a general purpose nucleophilic ion which we’ll call Nu-. . The bromoethane has a polar bond between the carbon and the bromine.
The lone pair on the Nu- ion will be strongly attracted to the + carbon, and will move towards it, beginning to make a co-ordinate (dative covalent) bond.
In the process the electrons in the C-Br bond will be pushed even closer towards the bromine, making it increasingly negative.
The movement goes on until the -Nu is firmly attached to the carbon, and the bromine has been expelled as a Br- ion.
What is a halogenoalkane?
An alkane with at least one halogen atom in place of a hydrogen atom.
What are CFCs?
Chloroflourocarbons (CFCs) are well known halogenoalkanes.
They’re very stable, volatile, non flammable and non toxic.
The carbon-halogen bond in halogenoalkanes is…?
Polar, this is because halogens are much more electronegative than carbon.
The delta + carbon doesn’t have enough electrons so it can’t be attacked by a nucleophile.
Halogenoalkanes can be hydrolysed to make what?
Alcohols.
What is the mechanism for bromoethane being hydrolysed to ethanol?
1) OH- is the nucleophile which provides a pair of electrons for the C delta+.
2) The C-Br bond breaks heterolytically- both electrons from the bond are taken by Br-.
3) Br- falls off as OH- bonds to the carbon.
How quickly different halogenoalkanes are hydrolysed depends on what?
Bond enthalpy.
Weaker carbon-halogen bonds break more easily- so they react faster.
Iodoalkanes have the weakest bonds, so they hydrolyse the fastest.
Fluoroalkanes have the strongest bonds, so they’re the slowest at hydrolysing.
What is the mechanism for halogenoalkanes?
The nucleophile, e.g a negatively charged OH- ion attacks the electron deficient carbon with a delta + charge.
The bond between teh halogen and carbon then breaks by heterolytic fission to create a hallide ion and a bond forms between the carbon and the OH-.
