6. Organic Molecules (HALOGENOALKANES)
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Describe the 3 types of halogenoalkanes and give them in order of their reactivity
Least reactive: Primary halogenoalkane - has 1 carbon attached to the carbon atom adjoining the halogen Middle: Secondary halogenoalkane - has 2 carbons attached to the carbon atom adjoining the halogen Most reactive: Tertiary halogenoalkane - has 3 carbons attached to the carbon atom adjoining the halogen By carbons –> mean alkyl groups
Give the 2 types of reactions that halogenoalkanes undergo
Either nucleophilic substitution, or elimination reactions
Define nucleophile
An electron pair donor –> attracted to areas of electron deficiency
Describe the conditions and reaction between water and halogenoalkanes via nucleophilic substitution
Produces an alcohol –> Need to heat halogenoalkane with water –> water is a weak nucleophile so reaction is slower than using hydroxide
Describe the conditions and products of the reaction between halogenoalkanes and aquous hydroxide ions
Produces an alcohol and the halogen with lone pair and negative charge Needs: —> warm aqueous sodium hydroxide (water could be used but its slower) —> under reflux
Describe the mechanism + reaction between haloalkanes and OH ions (eg: potassium hydroxide, sodium hydroxide)
–> OH- ion acts as a nucleophile and has lone pairs to donate –> OH- attacks delta carbon - draw curly arrow from lone pair on OH- to delta positive carbon –> The carbon - halogen bond breaks so both electrons from the original bond moves to the halogen to prouce the halogen with a lone pair - draw a curly arrow from the carbon - halogen bond to the halogen –> A new bond is formed between OH- ion and carbon to p[roduce an alchol
Give the trend for halogenoalkane reactivity down group 7 and how we can test for this (Core 4)
Halogenoalkanes become more reactive as the bond enthalpy (bond strength) decreases –> react chloroalkane, bromoalkane and iodoalkane with silver nitrate and ethanol. Whichever one forms precipitate the fastest is the most reactive
Core 4 questions: 1) If we want to investigate the rate of reaction of haloalkanes, why can’t we just use the reaction between water and haloalkanes? 2) Why is ethanol added to the haloalkane before silver nitrate is added? 3) What are some control factors? 4) Why do iodoalkanes have the fastest rate of reaction? 5) Why do tertiary haloalkanes have the fastest rate of reaction
1) There are two issues - all of the substances are colourless, so we don’t know when the reaction is finished; haloalkane and water are not soluble in one another so we have 2 layers so they wont react with each other Instead, we time how long they take to form a coloured precipitate using AgNO3 2) Ethanol is a co solvent so it will hydrogen bond with water and have London forces/ permanent dipole-dipole interactions 3) Use a water bath to control temperature 4) C-I is the longest bond, therefore the weakest bond and has the lowest bond enthalpy so breaks fast 5) Tertiary haloalkanes react via a tertiary carbocation which is more stable than a secondary or primary carbocation
Explain why iodobutane has a higher rate of reaction than chlorobutane
The C-I bond is longer than the C-Cl bond as the iodine atom is larger than a chlorine atom. This means the C-I bond is weaker than the C-Cl bond so needs less energy to overcome so the reaction is faster. This also means the C-I bond has a lower bond enthalpy
Why do nucleophiles attack halogenoalkanes
Halogenoalkanes have a polar bond. –> the halogen is more electronegative than carbon so they pull electrons towards themselves in a covalent bond so it becomes a polar bond –> nucleophile attracted to slightly positive carbon ( carbon is electron deficient) and replace halogen
Give some examples of nucleophiles
NH3 - ammonia OH- hydroxide ions CN- cyanide ions
Describe the condition and products of the reaction between cyanide ions and halogenoalkanes
Forms nitriles –> warm ethanolic potassium cyanide (KCN-) –> under reflux
Describe the condition and products of the reaction between ammonia and halogenoalkanes
Produces an amine (NH2) and ammonium chloride/iodide/bromide –> Heat with ethanolic ammonia –> Must have excess ammonia –> sealed container
Describe the mechanism for the reaction between ammonia and halogenoalkanes
1) ammonia (nucleophile) attacks carbon and replaces the halogen forming an intermediate –> Curly arrow from lone pair on ammonia to delta positive carbon. Curly arrow from carbon - halogen bond to the halogen 2) (N has a + by it) N now has 4 bonds instead of 3 so needs to lose a hydrogen 3) so another ammonia molecule (acting as a base) forms a dative bond with a hydrogen to form an ammonium ion –> Curly arrow from lone pair on ammonia to a hydrogen attached to +N –> curly arrow from that hydrogen and +N bond to the +N
How do you know an amine has been produced
Fishy smell
Describe the conditions for an elimination reaction between a hydroxide ion and halogenoalkane
Forms alkene, water and halogen ion –> ethanolic sodium hydroxide –> Under reflux
Describe the mechanism for elimination reaction between a hydroxide ion and halogenoalkane
1) OH- attacks the hydrogen on the carbon adjacent to the carbon with halogen on –> OH- acts as a base (proton acceptor) forming water –> Curly arrow from lone pair on OH- and hydrogen –> curly arrow from hydrogen to the other carbon with the halogen and delta positive 2) electrons in the bond move to form a double bond between the 2 carbons so therefore a bond must break. The weakest bond is the carbon - halogen bond so that bond breaks so the electrons from that bond move to the halogen –> forms alkene, water and halogen ion as the original hydrogen and halogen is eliminated
What does the solvent determine in halogenoalkane reactions with OH- ions?
If the reaction is elimination or nucleophillic substitution Ethanol - alkene is formed (elimination) Water - alcohol is formed (nucleophillic substitution) Both solvents = mix of both products