Haloalkanes David Martin City and Islington College

Topic Chemistry- Halogenoalkanes Aims  To help introduce students to Halogenoalkanes year olds Level Level 3 Method In-depth PowerPoint slides, ALL can be hand-outs, some slides have questions and answers (could be done as a group discussion) Equipment  Projector  Laptop  Pens  Hand-Outs Duration >30 Mins

Learning Outcomes What is a functional group? 1 st functional group – Haloalkanes (also known as Halogenoalkanes or Alkyl halides) Naming and Drawing Haloalkanes Isomers of Haloalkanes Physical Properties of Haloalkanes Chemical Properties of Haloalkanes

Functional Groups Functional groups are: a characteristic feature of organic molecules that behave in a predictable way composed of an atom or group of atoms groups that replace a hydrogen atom in the corresponding alkane a way to classify families of organic compounds

Haloalkanes Compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine).

The different kinds of Haloalkanes Halogenoalkanes fall into different classes depending on how the halogen atom is positioned on the chain of carbon atoms. There are some chemical differences between the various types. Primary Halogenoalkanes Secondary Halogenoalkanes Tertiary Halogenoalkanes

Primary Haloalkanes In a primary (1°) halogenoalkane, the carbon which carries the halogen atom is only attached to one other alkyl group. Notice that it doesn't matter how complicated the attached alkyl group is. In each case there is only one linkage to an alkyl group from the CH 2 group holding the halogen.

Secondary Haloalkanes In a secondary (2°) halogenoalkane, the carbon with the halogen attached is joined directly to two other alkyl groups, which may be the same or different.

Tertiary Halogenoalkanes In a tertiary (3°) Halogenoalkane, the carbon atom holding the halogen is attached directly to three alkyl groups, which may be any combination of same or different.

The reactions of Haloalkanes HaloalkaneObservation Tube 1 Chloro - alkanewhite precipitate Tube 2 Bromo - alkanevery pale cream precipitate Tube 3 Iodo - alkanevery pale yellow precipitate

Comparing the reaction rates as you change the Halogen A primary iodo compound produces a precipitate quite quickly A primary bromo compound takes longer to give a precipitate A primary chloro compound probably won't give any precipitate until well after you have lost interest in the whole thing! The order of reactivity reflects the strengths of the carbon-halogen bonds. The carbon-iodine bond is the weakest and the carbon-chlorine the strongest of the three bonds. In order for a halide ion to be produced, the carbon-halogen bond has to be broken. The weaker the bond, the easier that is

Physical Properties Boiling point Increases with molecular size due to increased van der Waals’ forces M r bp / °C chloroethane chloropropane bromopropane Boiling point also increases for “straight” chain isomers. Greater branching = lower inter-molecular forces bp / °C 1-bromobutaneCH 3 CH 2 CH 2 CH 2 Br bromobutaneCH 3 CH 2 CHBrCH bromo -2-methylpropane(CH 3 ) 3 CBr 73 Solubility Halogenoalkanes are soluble in organic solvents but insoluble in water

Substitution or Elimination? There are two different sorts of reaction that you can get depending on the conditions used and the type of Halogenoalkane. Primary, secondary and tertiary Halogenoalkanes behave differently in this respect.

Substitution Reactions In a substitution reaction, the halogen atom is replaced by an -OH group to give an alcohol. For example:

Elimination reactions Halogenoalkanes also undergo elimination reactions in the presence of sodium or potassium hydroxide. Alkene!

What decides whether you get Substitution or Elimination? 1.The type of halogenoalkane 2.The solvent 3.The temperature 4.Concentration of the sodium or potassium hydroxide solution

The type of Halogenoalkane type of halogenoalkane substitution or elimination? Primarymainly substitution Secondaryboth substitution and elimination Tertiarymainly elimination

In Summary…… For a given halogenoalkane, to favour elimination rather than substitution, use: higher temperatures a concentrated solution of sodium or potassium hydroxide pure ethanol as the solvent To favour substitution rather than elimination, use: lower temperatures more dilute solutions of sodium or potassium hydroxide more water in the solvent mixture

The nucleophilic substitution reactions between Halogenoalkanes and Hydroxide ions This is an example of nucleophilic substitution. Because the mechanism involves collision between two species in the slow step (in this case, the only step) of the reaction, it is known as an S N 2 reaction. Primary Haloalkene S = substitution N = Nucleophilic 2 = bimolecular (two molecules)

S N 2 reaction in detail

Continue…….. Tertiary Haloalkane This is again an example ofnucleophilic substitution. This time the slow step of the reaction only involves one species - the halogenoalkane. It is known as an S N 1 reaction.

Secondary Haloalkane S N 2 reaction S N 1 reaction Continue……..

The elimination reactions producing Alkenes from simple Halogenoalkanes The hydroxide ion removes a hydrogen from one of the carbon atoms next door to the carbon-bromine bond, and the various electron shifts then lead to the formation of the alkene - in this case, propene. Symmetrical haloalkane

Unsymmetrical Haloalkane To make room for the electron pair to form a double bond between the carbons, you would have to expel a hydrogen from the CH 2 group as a hydride ion, H -. That is energetically much too difficult, and so this reaction doesn't happen. That still leaves the possibility of removing a hydrogen either from the left- hand CH 3 or from the CH 2 group. Continue……..

If it was removed from the CH 3 group: If it was removed from the CH 2 group: but-1-ene but-2-ene Continue……..

Reactions between Halogenoalkanes and Ammonia a salt is formed - in this case, ethylammonium bromide reversible reaction excess ammonia The ammonia removes a hydrogen ion from the ethylammonium ion to leave a primary amine - ethylamine

ethylaminebromoethane excess ammonia reversible reaction The ammonia removes a hydrogen ion from the diethylammonium ion to leave a secondary amine - diethylamine. A secondary amine is one which has two alkyl groups attached to the nitrogen Continue……..

diethylaminebromoethane triethylammonium bromide salt excess ammonia reversible reaction The ammonia removes a hydrogen ion from the triethylammonium ion to leave a tertiary amine - triethylamine. A tertiary amine is one which has three alkyl groups attached to the nitrogen. Continue……..

triethylamine bromoethane a quaternary ammonium salt (one in which all four hydrogens have been replaced by alkyl groups). This time there isn't any hydrogen left on the nitrogen to be removed. The reaction stops here. Continue……..

Uses of Halogenoalkanes CFCs and their replacements What are CFCs? CFCs are chlorofluorocarbons - compounds containing carbon with chlorine and fluorine atoms attached. Two common CFCs are: CFC-11CCl 3 F CFC-12CCl 2 F 2

Uses of CFCs CFCs are non-flammable and not very toxic. They therefore had a large number of uses. They were used as refrigerants, propellants for aerosols, for generating foamed plastics like expanded polystyrene or polyurethane foam, and as solvents for dry cleaning and for general degreasing purposes. Unfortunately, CFCs are largely responsible for destroying the ozone layer. In the high atmosphere, the carbon-chlorine bonds break to give chlorine free radicals. It is these radicals which destroy ozone. CFCs are now being replaced by less environmentally harmful compounds.

CFCs can also cause global warming. One molecule of CFC-11, for example, has a global warming potential about 5000 times greater than a molecule of carbon dioxide. On the other hand, there is far more carbon dioxide in the atmosphere than CFCs, so global warming isn't the major problem associated with them. Continue……..

Replacements for CFCs These are still mainly Halogenoalkanes, although simple alkanes such as butane can be used for some applications (for example, as aerosol propellants). 1. Hydrochlorofluorocarbons, HCFCs HCFC-22CHClF 2 These have a shorter life in the atmosphere than CFCs, and much of them is destroyed in the low atmosphere and so doesn't reach the ozone layer. HCFC-22 has only about one-twentieth of the effect on the ozone layer as a typical CFC

2.Hydrofluorocarbons, HFCs These are compounds containing only hydrogen and fluorine attached to carbon. For example: HFC-134aCH 2 F-CF 3 Because these HCFCs don't contain any chlorine, they have zero effect on the ozone layer. HFC-134a is now widely used in refrigerants, for blowing foamed plastics and as a propellant in aerosols. 3.Hydrocarbons Again, these have no effect on the ozone layer, but they do have a down side. They are highly flammable and are involved in environmental problems such as the formation of photochemical smog. Continue……..

Other uses of organic Halogeno compounds In making plastics Chloroethene, CH 2 =CHCl, is used to make poly(chloroethene) usually known as PVC. Tetrafluoroethene, CF 2 =CF 2, is used to make poly(tetrafluoroethene) – PTFE.

Lab uses of Halogenoalkanes Halogenoalkanes react with lots of things leading to a wide range of different organic products. Halogenoalkanes are therefore useful in the lab as intermediates in making other organic chemicals.

Concept Map Haloalkanes SubstitutionElimination Reactions between Halogenoalkanes And ammonia

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