3 Haloalkanes (Paper 2) Flashcards Preview

3.3 Organic Chemistry > 3 Haloalkanes (Paper 2) > Flashcards

Flashcards in 3 Haloalkanes (Paper 2) Deck (15)
Loading flashcards...
1

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

2

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

3

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

4

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

5

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.

6

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

7

Do: Bromoethane + aqueous NaOH
Balanced equation (structural formulae)
Name of Organic product
Name of mechanism
mechanism

Look in booklet

8

Do: 2-bromopropane + ethanolic KOH
Balanced equation (structural formulae)
Name of Organic product
Name of mechanism
mechanism

Look in booklet

9

Do: Chloroethane + NH3
Balanced equation (structural formulae)
Name of Organic product
Name of mechanism
mechanism

Look in booklet

10

Do: Chloromethane + chlorine (w/UV light, excess of chloromethane)
Balanced equation (structural formulae)
Name of Organic product
Name of mechanism
mechanism

Look in booklet

11

Do: 1-chloropropane + KCN
Balanced equation (structural formulae)
Name of Organic product
Name of mechanism
mechanism

Look in booklet

12

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

13

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

14

Do initiation, prop and overall equation for decomp of ozone

Check FCs

15

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