3 Haloalkanes (Paper 2) Flashcards
Nucleophilic substitution: what are nucleophiles, electronegativity, examples, bonds, ROR- high to low etc
Haloalkanes are susceptible to attack by nucleophiles (lone pair donors, positive seekers) e.g :OH-, :CN-, :NH3
This is bcuz the halogen atom is more electronegative than carbon atoms and so the C of the C-halogen bond is δ+, halogens δ-
In a substitution reaction, the halogen atom is replaced by another atom/group (the nucleophile group)
ROR partly affected by strength of C-halogen bond. Longer the bond, weaker the bond, the more easily it breaks and the faster the reaction.
∴in terms of ROR: C-I > C-Br > C-Cl > C-F
Nucleophilic substitution: Reaction NaOH- conditions, reagent, what happens, overall equation, do examples: 2-chloropropane and bromoethane
Reagent: NaOH (ignore Na)
Conditions: Warm, aqueous NaOH
What happens: Halogen replaced by OH group
Overall equation: R-X + NaOH→ R-OH + NaX
R- rest of alkane, alkyl grp
X- Halogen
Nucleophilic substitution: Reaction KCN- conditions, reagent, what happens, overall equation, do examples: 2-chloropropane
Reagent: KCN
Conditions: ethanolic, warm
What happens: Halogen replaced by CN group
Overall equation: R-X + KCN→ R-CN + KX
Nucleophilic substitution: Reaction NH3- conditions, reagent, what happens, overall equation, do examples: 2-chloropropane
Reagent: (2)NH3
Conditions: excess conc. ammonia dissolved in ethanol at pressure in sealed container
What happens: First molec NH3- halogen replaced by NH2 grp
Second molec of NH3- leads to formation of NH4X
Overall equation: R-X + 2NH3→ R-NH2 + NH4X
Elimination: what is it, when favoured, what does OH- act as, where atoms are removed from
When halogenoalkanes react with OH ions, an elimination reaction can compete with
the nucleophilic substitution reaction.
Elimination is favoured if HOT, ETHANOLIC KOH is used instead of warm, aqueous NaOH.
In elimination, an H and X are removed from ADJACENT C atoms giving an alkene.
In elimination, the OH ion acts as a BASE.
In substitution, the OH ion acts as a
nucleophile.
Elimination: Reaction KOH- conditions, reagent, what happens, overall equation, do examples: 2-chloropropane
Reagent KOH
Conditions: ethanolic, hot
What happens: The halogen atom and one H atom from an adjacent C atom is removed giving an
alkene (note that elimination cannot happen if there is no H on an adjacent C atom).
A mixture of alkenes could be formed depending on which of the adjacent C atoms the
H is lost from.
I I I
Overall Eq: -C-C + KOH = -C=C- + KX + H2O
I I
H X
Do: Bromoethane + aqueous NaOH Balanced equation (structural formulae) Name of Organic product Name of mechanism mechanism
Look in booklet
Do: 2-bromopropane + ethanolic KOH Balanced equation (structural formulae) Name of Organic product Name of mechanism mechanism
Look in booklet
Do: Chloroethane + NH3 Balanced equation (structural formulae) Name of Organic product Name of mechanism mechanism
Look in booklet
Do: Chloromethane + chlorine (w/UV light, excess of chloromethane) Balanced equation (structural formulae) Name of Organic product Name of mechanism mechanism
Look in booklet
Do: 1-chloropropane + KCN Balanced equation (structural formulae) Name of Organic product Name of mechanism mechanism
Look in booklet
What is the ozone layer?
Ozone (O3) found w/in stratosphere
Important bc it absorbs UV(B) from the sun which protects life on Earth from harmful effects of UV like sunburn, genetic damage and cancer
One covalent one dative covalent
What are CFCs? Why are they a problem? What properties make them desirable, what are their uses? Why might one property be a problem?
Chlorofluorocarbons
Problem bc they rise to stratosphere and UV light then provides energy that is used to break C-Cl bond- form Cl• radicals that catalyse decomposition of ozone
Properties: stables (C-X bond v strong- until reach upper atmosphere then brkdown), volatile (easily vaporise) and non-flammable, non-toxic
Uses: (CF3Br) Flame retardents (CHCl3) Solvents, coolants in fridges, pipework
V long life so still entering atmosphere today
Do initiation, prop and overall equation for decomp of ozone
Check FCs
What is being done (or not being done) to fix problem of ozone depletion, what is an alternative and why does it work
Done: Montreal Protocall 1987- 24 countries banned CFCs
BUT: Still used by lots of countries
CFCs take long time to reach atmosphere so still entering
Still lots of CFCs stored in old fridges etc
HFCs- no Cl so no ozone depletion potential
C-F: High bon enthalpy so NOT broken by UV light
C-Cl- weaker bond (lower en) so CAN be broken by UV light
