Alkyl Halides

Alkyl halides

organic compounds in which one or more halogen atoms- fluorine, chlorine, bromine, or iodine- are substituted for one or more hydrogen atoms in a hydrocarbon

Primary alkyl halide

An alkyl halide in which the carbon atom bonded to the halogen is also bonded to 1 other carbon atom.

Secondary alkyl halide

an alkyl halide in which the halogen is bonded to a secondary carbon

Tertiary alkyl halide

an alkyl halide in which the halogen is bonded to a tertiary carbon

Halogenoalkane

An alkane with at least one halogen atom in place of a hydrogen atom

CFCs

Chlorofluorocarbons.

Non flammable, not toxic.

Used in refrigerants, aerosols, propellants, plastics, degreasing.

Dangers of CFC

Destroy the ozone layer.

Carbon-chlorine bond breaks into chlorine free radicals, which break down ozone.

CFC-11

5000x greater global warming potential that CO2

Halothanes

Only inhalation anesthetic agent containing a bromine atom.

CCl4 uses

Used in fire extinguishers,

precursor to refrigerants,

cleaning agent.

Halothanes characteristics

Colorless,

Pleasant smelling,

Unstable in light

CCl4 (Carbon tetrachloride) characteristics

Colorless liquid,

sweet smell that can be detected at low levels

CCl4 other names

Halon

Freon

in HV AC

CHCl3 (Chloromethane) uses

Used to be used as a refrigerant, but not anymore because it is toxic.

Used for making lead based additives for gasoline.

Chemical intermediate in the production of silicone polymers.

Solvent in the manufacture of butyl rubber.

Methylating and chlorinating agent.

Many other uses

Teflon (PTFE) uses

Non stick coating for pans,

Containers and pipework for reactive and corrosive chemicals.

Lubricant for machinery

Teflon (PTFE) characteristics

Non reactive because of the strength in the carbon-fluorine bonds.

What does the reactivity of alkyl halides depend upon?

C-X bond energy and polarity

The greater the bond energy of R-X,

The greater the stability,

The less reactivity

The greater the electronegative difference (bond polarity) of R-X,

The greater will be the stability and the lesser the reactivity

Reactivity of halogens in order

I > Br > Cl > F

When is R-F the most reactive?

If an electrophile is attacking the reagent.

Which two types of reactions do alkyl halides show generally?

Nucleophilic substitution,

Elimination

Nucleophilic substitution reaction

-SN1 and SN2

- Reactions in which the halogen is replaced by some other atoms or a group

Elimination reaction

The removal of HX from the halide,

E reactions

Electrophile

Electron deficient molecules which accept electron pairs to make new covalent bonds

Examples of electrophiles

Positive ions,

Neutral molecules (SO3, AICI3, BF3),

Carbon atom bearing positive charge in a partially ionic covalent molecule

Nucleophiles

Electron rich molecules which donate electron pairs

Examples of nucelophiles

Negative ions,

Molecules with lone pairs (NH3, H2O, -NH2),

A molecule with a pi electron (C2CH, C2H4),

Carbon bearing partial negative charge in organometallic compounds (Grignard's reagent)

Leaving group

The group that departs with an unshared pair of electrons.

Sn reactions, the incoming nucleophile must be stronger than the leaving one.

Sn reactions

Nucleophilic substitution reactions

Which two ways can Sn reactions occur?

Sn1: Nucleophilic substitution unimolecular,

Sn2: Nucleophilic substitution bimolecular

Sn 1 reaction molecularity

1

Molecularity

the number of molecules that participate as reactants in an elementary reaction

Sn 1 reaction mechanism

2 step mechanism.

R-X --> R+ X- (slow),

R+ + Nu- --> R--Nu (fast)

Sn 1 reaction rate

Rate α [R-X]

Rate = k[R-X]

Sn 1 reaction order

1

Sn 1 reaction

In tertiary alkyl halides.

Coming nucleophile may attack from any side.

Favored in polar solvents

Sn 2 reaction molecularity

2

Sn 2 reaction mechanism

Single step.

R-X + Nu- --> R-Nu + X- (slow)

Sn 2 reaction rate

Rate α [R-X][Nu-]

Rate = k[R-X] [Nu]

Sn 2 reaction order

2

Sn 1 reaction product

Racemic mixture.

50% inversion

50% retention of configuration

Sn 2 reaction product

100% inversion of configuration

Sn 2 reaction

In primary alkyl halides.

Coming nucleophile attacks from backside.

Favored in non polar solvents.

What kind of reactions does a secondary alkyl halide give?

Sn 1 and 2, depending on the structure of alkyl group or nature of the solvent

Processes in Sn reactions

1) Breakage of C-L or C-X bond.

2) Formation of C-Nu bond.

What does the mechanism of the Sn reaction depend on?

Timing of the 2 main processes.

If the processes occur simultaneously, its Sn2.

If the bond breaks first and then the formation happens, it is Sn1.

When purely alcoholic solution of sodium/potassium hydroxide and halogenoalkanes are reacted an alkene is formed, what is the mechanism of reaction

Elimination

The organic compound carbon tetrachloride is used as

Solvent

Hydrolysis of bromoethene condition

Dilute NaOH warm

In substitution reactions, dihaloalkane or secondary halogenoalkane give / show

Both SN1 and SN2

The order of reactivity of alkyl halides towards nucleophile is

RI > RBr > RCl > RF

The halothane used in hospitals as an anesthetic is chemically

2-Bromo-2-chloro-1, 1, 1-trifluroethane

Is BF3 a nucleophile?

No