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Halogenoalkanes and Alcohols
Halogenoalkanes
Halogenoalkanes contain a halogen as the functional group R-Hal.
The first part of the name of an halogenoalkane gives the position and name of the halogen concerned. The second part of the name is based on the rest on the hydrocarbon structure.
H C C C C
H
H
H
H
H
H
Cl
H
H
H C C C C
H
H
H
H
H
H
C
H
Br
H
H
H 1-chlorobutane 2-Bromopentane
H C C C C
H
H
I
C
H
H
H
H
H
H H (^) H 2-Iodo-2-methylbutane
2,3-Dichloroheptane 3-Bromo-3-methylhexane
Types of Halogenoalkanes
There are three types of halogenoalkane; primary, secondary and tertiary. They are classified according to the number of carbon groups attached to the carbon with the halogen, X, group.
Primary Secondary Tertiary
H
H
H
C X R
H
H
C X R
H
R
C X R
R
R
C X
No or one carbon (R) group attached to the carbon with the X group is a primary halogenoalkane.
Two carbon (R) groups attached to the carbon with the X group is a seconary halogenoalkane.
Three carbon (R) groups attached to the carbon with the X group is a tertiary halogenoalkane
Cl
Cl
Br
Chemical properties of Halogenoalkanes
Fluoroalkanes are different from the other halogen derivatives, since the C–F bond is so strong that they are very unreactive. Most of the discussion here will refer to RCl, RBr and RI.
Chloromethane, bromomethane, and chloroethane are gases, while the rest are liquids which do not mix with water as they do not have the ability to hydrogen bond.
Preparation of Halogenoalkanes
Halogenoalkanes are generally made by reacting the appropriate alcohol it with a halogenating reagents. Halogenating agents include phosphorus pentachloride, sodium chloride with concentrated sulphuric acid, sodium bromide with concentrated phosphoric acid and phosphorus with iodine.
Phosphorus pentachloride reacts vigorously with alcohols at room temperature. C 2 H 5 OH + PCl 5 C 2 H 5 Cl + HCl + POCl 3
Choro- compounds can also be formed by heating the alcohol under reflux with sodium chloride and concentrated sulphuric acid. H 2 SO 4 + NaCl HCl + NaHSO 4 HCl + C 3 H 7 OH C 3 H 7 Cl + H 2 O
To form a bromo- compound, the alcohol is heated under reflux with sodium bromide and concentrated phosphoric acid. The concentrated phosphoric acid reacts with the sodium bromide to form hydrogen bromide, and the hydrogen bromide carries out the substitution. H 3 PO 4 + NaBr HBr + NaH 2 PO 4 HBr + C 3 H 7 OH C 3 H 7 Br + H 2 O
To produce iodo- compounds, the alcohol is mixed with red phoshorus and iodine is added gradually. The mixture is then heated under reflux. P + 1½I 2 PI 3 PI 3 + 3CH 3 CH(OH)CH 3 3CH 3 CHICH 3 + H 3 PO 3
Concentrated sulphuric acid cannot be used to make bromoalkanes or iodoalkanes as the halide ion is oxidized to the halogen.
Identification of halogenoalkanes – Reaction with Silver nitrate solution.
In the presence of water, halogenoalkanes undergo hydrolysis. The halogenoalkanes have a slightly polarised C-Hal bond. Water acts as a nucleophile towards the carbon atom in this bond. As a result , the -OH group substitutes for the halogen, giving an alcohol and a hydrogen halide.
The reaction is much slower than with an alkali. RX + H 2 O ROH + HX
The hydrogen halide formed will dissolve in the water forming H+^ and X-^ ions. The ions then react with the silver ions in the solution producing a precipitate. The appearance of the precipitate depends upon the halide ion generated in the hydrolysis reaction. Ag+(aq) + X-(aq) AgX(s)
This reaction is used to test for halogenoalkanes.
- Heat sample of halogenoalkane with aqueous hydroxide ions.
- Acidify with dilute aqueous nitric acid.
- Add a few drops of aqueous silver nitrate.
- A white precipitate soluble in dilute aqueous ammonia, indicates chloride.
- A buff precipitate insoluble in dilute aqueous ammonia but soluble in concentrated aqueous ammonia, indicates bromide.
- A yellow precipitate insoluble in concentrated aqueous ammonia indicates iodide.
Reactivity of halogenoalkanes
Results of investigations show that the rate of hydrolysis of the halogenoalkanes occurs in the order: 1-iodobutane > 1-bromobutane > 1-chlorobutane
The ease of reaction depends on the ease of breaking the C-Hal bond:
Bond : C-I C-Br C-Cl C-F Bond enthalpy terms (kJ mol-1^ ): +238 +276 +338 +
Thus the ease of bond breaking is, C-I > C-Br > C-Cl > C-F.
(This outweighs the effects caused by greater polarization in the C-Hal bond).
Uses of halogenoalkanes
Halogenoalkenes are use as fire retardants and refrigerants as they are unreactive and
particularly for the fire retardants they are non-combustible.
Freon 12 CF 2 Cl 2 is a refrigerant and an example of a chlorofluorcarbon (CFC). The C-F and C-Cl bonds are very strong. The result is that it does not decompose easily so lasts for the lifetime of a refrigerator. It does not decompose quickly when discarded but does so in the upper atmosphere. The radicals it forms react with ozone. The loss of ozone leads to an increase in UV radiation reaching the Earth's surface and a corresponding increase in skin cancers in humans. Modern refrigerants are hydrofluorocarbons, HFCs, such CF 3 CH 2 F. They do not lead to ozone depletion.
PVC used as electrical insulator -[-CH 2 -CHCl-]n- The C-Cl bond is strong so PVC insulation lasts a long time but when discarded it does not rot (it is not biodegradable).
Teflon, or poly(tetrafluoroethene) is essentially poly(ethene) chains in which all the H atoms have been replaced by fluorines: –CF 2 –CF 2 –CF 2 –CF 2 –CF 2 –CF 2 –CF 2 –CF 2 –CF 2 – etc. It is used to line non-stick frying pans and saucepans , and for low-friction bearings. As the C-F is very strong it is non-biodegradable.
DDT is a pesticide used to kill mosquitos. CCl 3 | Cl-C 6 H 5 -C-C 6 H 5 -Cl | H
The strong C-Cl bonds give DDT a long life in the field killing pests. It is however so long lived that it persists in the environment and builds up in the food chain threatening creatures at the top of the chain.
Physical properties of alcohols
The OH group can take part in hydrogen-bonding, both as a donor (H is sufficiently δ+) and as an acceptor (through the two lone pairs on the oxygen atom). As a result alcohols have higher melting and boiling points than hydrocarbons of comparable molar mass.
They are also more soluble in water because of H-bonding: ethanol is miscible with water in all proportions. As the non-polar hydrocarbon chain becomes longer, it becomes harder for water to dissolve the alcohol: from C 4 alcohols are less soluble, and don’t mix with water.
Reactions of Alcohols
Combustion All alcohols undergo combustion to form carbon dioxide and water. For example the equation for the combustion of butanol is as follows; C 4 H 9 OH + 6O 2 4CO 2 + 5H 2 O
Reaction with sodium All alcohols react with sodium. 2ROH + 2Na 2RONa + H 2 This equation is similar to the reaction of sodium with water, except that an alkoxide is formed rather a hydroxide.
e.g. Ethanol and sodium 2C 2 H 5 OH + 2Na 2 C 2 H 5 ONa + H 2 Sodium ethoxide
Reaction with phosphorus pentachloride All alcohols react with phosphorus pentachloride. This is used as a test for the -OH group. The presence of the OH group can be shown by adding phosphorus pentachloride to the compound. A reaction takes place forming hydrogen chloride which appears as steamy white fumes. ROH + PCl 5 RCl + POCl 3 + HCl In the reaction with phosphorus pentachloride a chloro group replaces the -OH group.
e.g. Propanol and phosphorus pentachloride CH 3 CH 2 CH 2 OH + PCl 5 CH 3 CH 2 CH 2 Cl + POCl 3 + HCl
Reaction with other halogenating agents Alkyl bromides can be made from the reaction of an alcohol with HBr. HBr is made in situ from KBr and H 2 SO 4 forms bromoalkanes during heating under reflux. C 2 H 5 OH + HBr C 2 H 5 Br + H 2 O
Alkyl chlorides can be made by refluxing the alcohol with conc. HCl in the presence of ZnCl 2 C 2 H 5 OH (l) + HCl (g) C 2 H 5 Cl (l) + H 2 O (l)
Iodoalkanes can be made in a reaction phosporus triiodide from iodine and red phosporus. 3C 2 H 5 OH + PI 3 3C 2 H 5 I + H 3 PO 3 The relative reactivities of alcohols in halogenation are tertiary > secondary > primary alcohol.
Oxidation of alcohols
Primary and secondary alcohols can be oxidised by heating with a mixture of dilute sulphuric acid with sodium or potassium dichromate(VII) solution.
Acidified dichromate(VI) solution is produces a colour change from orange to blue-green when it has undergone oxidation reactions.
In writing equations for these oxidation reactions [O] is used to represent the oxidising agent.
Primary alcohols form an aldehyde, and then on further oxidation, form carboxylic acids.
Primary alcohol Aldehyde Carboxylic acid
Secondary alcohols form ketones, but no further oxidation takes place.
Secondary alcohol Ketone
e.g. CH 3 CH(OH)CH 3 + [O] CH 3 COCH 3 + H 2 O propan-2-ol propanone
Tertiary alcohols do not react with oxidizing agents.
R H
H
H
C O C
H
O
R C
O
R
O H
R H
H
R
C O C
R
O
R
Once the carboxylic has been formed, it needs to be separated from the reaction mixture and other products. This is done by distillation.
Distillation is used to separate a volatile product from a mixture of involatile substances, or substances that have a boiling point of at least 50o^ C higher than the component being collected.
Heat
