halogenoalkanes 3.3.3 in papers 2&3
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how do you go about naming halogenalkanes?
• identify the longest chain • identify the groups attached and place them in alaphabetical order. (fluoro, chloro, bromo or iodo) • identify the number and location of each halogen, if multiple use di, tri, tetra, penta etc.
why is iodoethane more reactive than a normal ethane alkane?
• contains a polar bond. • C-I is polar due to iodine being more electronegative than carbon. • therefore meaning that the carbon attached to the iodine will be susceptible to attack by species that are electron rich or have a high electron density.
what are important rules to remember when drawing mechanisms?
• curly arrows must be used. • the curly arrows must always start in the middle of a bond or lone pair. • the curly arrow must always end on an atom, ion or half way between 2 atoms.
what nucleophiles do halogenoalkanes undergo subsitution with and whats the product of them?
• 0H-, alcohol • CN- (cyanide ion), nitriles • NH3, amine
what is a nucleophile?
a species that donates an electron pair (to form a new covalent bond)
what is an electrophile?
a species that accepts an electron pair (to form a new covalent bond).
explain a nucleophilic subsitution with NaOH.
• OH- • NaOH dissolved in water/ aqueous ethanol, 60 degrees. • alcohol is produced.
explain the nucleophilic subsitution reaction with KCN.
• CN- • ethanol solvent, 60 degrees no water, only way to dissolve it as its soluble. • nitrile is produced. • the carbon change legnth is increased by 1.
explain the nucleophilic subsitution reaction with NH3.
• :NH • ethanol solvent, 60 degrees. • the ammonia must be in excess because some of it is needed to act as a nucleophile and some is needed to be a based. (excepts H+) • amine is produced.
what is hydrolysis?
• the break down of a bond using water. • cold water slowly hydrolyses halogenalkanes, replacing the halogen atoms with -OH to form alcohols.
which halogenalkane is the most reactive and why?
• iodobutane , then bromo and chloro. • the C-I bond has the lowest bond enthalpy comapred to the others. • meaning that the C-I bond is the most easily broken down by water. • this is because as you go down group 7, the bonding pair of elctrons gets further and futher away from the nucleui, making the electrostatic forces weaker.
why is ethanol added in the test for halogenalkanes?
to dissolve the halogenoalkane, on their own, halogenoalkanes arent soluble in water.
what conditions are required for halogenoalkanes to undergo elimination?
• high tempretures • have ethanol and no water
what is an elimination reaction?
• removal of small molecule (often water) from the organic molecule.
explain elimination with hydroxide ions.
• in ethanol and high heat • alcoholic OH making it elimination • OH base is used
why is a free radical subsitution reaction likely to give a low yield of the desired product?
• radicals are highly reactive and so could form a range of other halogenalkanes • could give rise to subsitution in different positions