Halogenoalkanes (Alkyl Halides) (Senior 5 Chemistry): free lessons | Ganzaa

Halogenoalkanes (Alkyl Halides)

Lessons

Senior 5 · Chemistry

Halogenoalkanes (Alkyl Halides)

4 lessons

Lesson 1 of 4

Halogenoalkanes: Names, Classes and Physical Properties

22 min

The solvent in the brown bottle

Many school laboratories keep a heavy, colourless liquid that does not mix with water and sinks below it: dichloromethane, $\mathrm{CH_2Cl_2}$ . It dissolves fats, paints and caffeine, and was once used to decaffeinate coffee. Replace a hydrogen atom of an alkane with a halogen atom and the compound changes completely: it becomes denser, less flammable, a better solvent and, unlike the alkane, reactive. That reactivity has made halogenoalkanes some of the most useful compounds in industry, and some of the most damaging to the environment.

Definition

Halogenoalkane (alkyl halide): a compound in which one or more hydrogen atoms of an alkane have been replaced by halogen atoms (F, Cl, Br, I); with one halogen, the general formula is $\mathrm{C_nH_{2n+1}X}$ , written R–X. Example: bromoethane, $\mathrm{CH_3CH_2Br}$ .

Primary (1°), secondary (2°), tertiary (3°) halogenoalkane: the carbon atom carrying the halogen is bonded to one, two or three other carbon atoms. Example: 2-bromopropane is secondary.

Nucleophile: a species with a lone pair of electrons that it donates to an electron-poor atom to form a new covalent bond. Examples: $\mathrm{OH^-}$ , $\mathrm{CN^-}$ , $\mathrm{NH_3}$ , $\mathrm{H_2O}$ .

Names, classes and isomers

The halogen is named as a prefix (fluoro-, chloro-, bromo-, iodo-) with its locant, and all prefixes go in alphabetical order with the alkyl branches: $\mathrm{CH_3CHBrCH(CH_3)_2}$ is 2-bromo-3-methylbutane. Common names are still met: chloroform for trichloromethane, $\mathrm{CHCl_3}$ , and methylene chloride for dichloromethane.

Figure 1.1: The four structural isomers of C₄H₉Br and their classes. 2-bromobutane also has a pair of enantiomers.

Halogenoalkanes show chain and position isomerism, and also optical isomerism when the C–X carbon carries four different groups, as in 2-bromobutane. The class matters because primary, secondary and tertiary halogenoalkanes react by different mechanisms (Lesson 3).

The C–X bond

Figure 1.2: The C–X bond is polar (left), and its strength falls from C–F to C–I (right; typical data-book values, which vary slightly between sources).

Each halogen is more electronegative than carbon (Pauling values: C 2.55, F 3.98, Cl 3.16, Br 2.96, I 2.66), so the C–X bond is polar, $\mathrm{C^{\delta+}{-}X^{\delta-}}$ . The $\delta+$ carbon attracts nucleophiles. Down the group the halogen atom gets bigger, the bond gets longer (C–F 139 pm, C–Cl 177 pm, C–Br 194 pm, C–I 214 pm) and weaker. As you will see, it is bond strength, not polarity, that decides how fast halogenoalkanes react.

Physical properties

Halogenoalkane	Boiling point / °C	Halogenoalkane	Boiling point / °C
chloroethane	12	1-chlorobutane	78
bromoethane	38	1-bromobutane	102
iodoethane	72	1-iodobutane	131
ethane (for comparison)	−89	2-bromo-2-methylpropane	73

Boiling points are much higher than those of the parent alkanes: the halogen adds many electrons (stronger London forces) and a permanent dipole. They rise from Cl to I and with chain length, and fall with branching.
Solubility: although polar, halogenoalkanes cannot form hydrogen bonds with water, so they are almost insoluble in it. They dissolve in, and are themselves, good solvents for organic substances.
Density: bromo- and iodoalkanes and compounds with several chlorine atoms are denser than water (bromoethane 1.46, dichloromethane 1.33 $\mathrm{g\,cm^{-3}}$ ), so they form the lower layer in a separating funnel.
State: chloromethane, bromomethane and chloroethane are gases at room temperature; most others are liquids.

Worked example

Problem. Name each compound and classify it as 1°, 2° or 3°: (a) $\mathrm{CH_3CHClCH_3}$ ; (b) $\mathrm{(CH_3)_3CCl}$ ; (c) $\mathrm{BrCH_2CH(CH_3)CH_3}$ ; (d) $\mathrm{CH_3CH_2C(CH_3)BrCH_2CH_3}$ .

Solution. (a) 2-chloropropane, secondary (the C–Cl carbon has two C neighbours). (b) 2-chloro-2-methylpropane, tertiary. (c) 1-bromo-2-methylpropane, primary (the $\mathrm{CH_2Br}$ carbon has one C neighbour, even though the molecule is branched). (d) Five-carbon chain with Br and a methyl group both on C3: 3-bromo-3-methylpentane, tertiary.

Worked example

Problem. 1-bromobutane boils at 102 °C, but its isomer 2-bromo-2-methylpropane boils at 73 °C. Bromoethane boils at 38 °C. Explain both differences.

Solution. The two $\mathrm{C_4H_9Br}$ isomers have the same number of electrons, so their London forces differ only through shape: the compact, branched 2-bromo-2-methylpropane has less surface contact with its neighbours, so weaker intermolecular forces and a lower boiling point. Bromoethane has two fewer carbons, and so fewer electrons than either: weaker London forces and a lower boiling point still.

Warning

Classify by the carbon that carries the halogen, not by the shape of the whole molecule. 1-bromo-2-methylpropane is branched but primary; 2-bromobutane is unbranched but secondary.

Tip

Count the carbons touching the C–X carbon: one → 1°, two → 2°, three → 3°. (Halomethanes such as $\mathrm{CH_3Br}$ , with none, behave like primary ones.)

Alt Academy: Physical Properties of Halogenoalkanes, CAIE AS Level Chemistry (14 min). Watch how boiling point depends on the halogen and on chain length, and why halogenoalkanes do not dissolve in water.

✦

Practice

Name: (a) $\mathrm{CH_3CHBrCH_2CH_3}$ ; (b) $\mathrm{CHCl_3}$ ; (c) $\mathrm{CH_2ClCH_2Cl}$ ; (d) $\mathrm{(CH_3)_2CICH_2CH_3}$ .
Classify each compound in question 1 that has one halogen atom as 1°, 2° or 3°.
Name the four structural isomers of $\mathrm{C_3H_6Cl_2}$ . Which of them is chiral?
Explain why the C–Cl bond is polar, and state which end is $\delta+$ .
Put in order of increasing boiling point: iodoethane, chloroethane, bromoethane. Explain.
Explain why bromoethane does not dissolve in water, although its molecules are polar.
A mixture of water and dichloromethane is shaken in a separating funnel. Which liquid forms the lower layer? Why?

Show answersHide answers

(a) 2-bromobutane; (b) trichloromethane; (c) 1,2-dichloroethane; (d) 2-iodo-2-methylbutane.
(a) secondary; (d) tertiary.
1,1-dichloropropane, 1,2-dichloropropane, 1,3-dichloropropane, 2,2-dichloropropane. 1,2-dichloropropane is chiral: C2 carries H, $\mathrm{CH_3}$ , Cl and $\mathrm{CH_2Cl}$ .
Chlorine is more electronegative than carbon, so it pulls the bonding electrons towards itself: C is $\delta+$ , Cl is $\delta-$ .
Chloroethane (12 °C) < bromoethane (38 °C) < iodoethane (72 °C): the larger halogen has more electrons, giving stronger London forces.
It cannot form hydrogen bonds with water; the attractions it could form with water molecules are too weak to make up for breaking the hydrogen bonds between them.
Dichloromethane: its density (1.33 $\mathrm{g\,cm^{-3}}$ ) is greater than that of water.

Sources and credits

Written by Ganzaa.
Image: Sample of dichloromethane.jpg by LHcheM, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Video: Physical Properties | Halogenoalkanes | CAIE AS Level Chemistry 9701, Alt Academy, embedded from YouTube.
Bond enthalpies, bond lengths, electronegativities, boiling points and densities are typical data-book values.
The text, diagrams, table, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1The solvent in the brown bottle

2Names, classes and isomers

3The C–X bond

4Physical properties

The solvent in the brown bottle

Names, classes and isomers

The C–X bond

Physical properties