Did you know that approxiamtely one-fifth of all pharmecutical drugs contain fluorine? This element is mostly found in some sort of alkyl fluoride, an example of a halogenoalkane.
- This article is about halogenoalkanes in organic chemistry.
- We'll define halogenoalkane before looking at halogenoalkane nomenclature.
- We'll then learn how you classify halogenoalkanes as primary, secondary or tertiary.
- After that, we'll focus on properties of halogenoalkanes.
- It will then be time to explore production of halogenoalkanes, followed by their reactivity.
- This will involve learning about nucleophilic substitution reactions.
- Finally, we'll end by discussing uses of halogenoalkanes.
Halogenoalkane definition
Halogenoalkanes are organic molecules formed from alkanes, where halogen atoms have replaced one or more hydrogen atoms.
Halogenoalkanes are also known as haloalkanes or alkyl halides. Simply put, they are alkanes, but they contain a halogen atom instead of one (or more!) of their hydrogen atoms. The halogen atom is referred to as X.
As you might know from Halogens, halogens are elements that belong to group 7 of the periodic table. This group is also known as group 17. The halogens all have seven electrons in their outer shell and tend to have high electronegativities. Fluorine is the smallest member of the group, whilst the largest is astatine. However, astatine is radioactive and short-lived, so not commonly used.
Halogenoalkane general formula
The general molecular formula for a halogenoalkane with a single halogen atom is CnH2n+1X. Examples include chloroethane, C2H5Cl, and bromomethane, CH3Br.
Chloroethane, left, and bromomethane, right, with the halogen highlighted. Vaia Originals
Halogenoalkane nomenclature
Halogenoalkanes are named using standard nomenclature rules and the appropriate prefix, as shown in the table below.
| Halogen present | Prefix |
| Fluorine | fluoro- |
| Chlorine | chloro- |
| Bromine | bromo- |
| Iodine | iodo- |
If you’re not familiar with the basics of naming organic molecules, take a quick look at Organic Compounds. But for those of you who feel confident, let’s use some examples to practice applying our knowledge of nomenclature.
Name the following molecule:
An unknown halogenoalkane. Vaia Originals
To start, we can see that this molecule has four carbon atoms. It therefore has the root -butane. It also has a chlorine atom attached to one of the carbons and so will start with the prefix chloro-. You’ll know that the positions of functional groups on the carbon chain numbers are indicated with numbers. We number the carbon chain both from the left and from the right, and try to make sure that the functional group takes the lowest number possible. Here, the chlorine is attached to either carbon 2 or 3, depending on where you start counting. 2 is lower than 3, so we call this molecule 2-chlorobutane.
Our unknown halogenoalkane, numbered correctly (green) and incorrectly (red). Vaia Originals
Number the carbon chain from both directions so the functional group is attached to the carbon atom with the lowest number.
Here’s another example:
Name this halogenoalkane:
A second unknown halogenoalkane. Vaia Originals
We can see that this molecule has four carbon atoms and two functional groups: a fluorine atom and a chlorine atom. This gives it the suffix -butane and the prefixes chloro- and fluoro-. You should remember that if there are two functional groups present, we list them in alphabetical order. However, the numbering rule still applies - if we add the numbers before each functional group, we want the lowest total possible. Let’s number the carbon chain now.
Our mystery second unknown halogenoalkane, numbered correctly (green) and incorrectly (red). Vaia Originals
The function groups are either present on carbons 1 and 3, or 2 and 4. 1 + 3 = 4, whereas 2 + 4 = 6. 4 is a lower total than 6. In this case, we would therefore number the carbon chain from right to left, so the functional groups are attached to carbons 1 and 3. If we put that all together, we get the name 3-chloro-1-fluorobutane.
Primary, secondary, and tertiary halogenoalkanes
Halogenoalkanes are classified as primary, secondary, or tertiary. This depends on the number of alkyl groups attached to the C-X bonded carbon. Classification is referred to using the degree symbol (°), as shown below. Alkyl groups are often called R groups when discussing organic molecules.
- In a primary halogenoalkane (1°), the carbon bonded to the halogen atom is also bonded to either zero or one R group.
- In a secondary halogenoalkane (2°), the carbon bonded to the halogen atom is also bonded to two R groups.
- Likewise, in a tertiary halogenoalkane (3°), the carbon bonded to the halogen atom is also bonded to three R groups.
Here are some examples of primary, secondary, and tertiary halogenoalkanes. We've highlighted the halogen atom in blue and the R groups in red, to help you identify them within the molecule.
Examples of halogenoalkane classification. Vaia Originals
Properties of halogenoalkanes
Halogenoalkanes have slightly different properties to alkanes due to their polar C-X bond. This is all thanks to the different electronegativities of carbon and the halogens, which are shown in the table below.
| Element | Electronegativity |
| Carbon | 2.5 |
| Fluorine | 4.0 |
| Chlorine | 3.5 |
| Bromine | 2.8 |
| Iodine | 2.6 |
You can see that all of the halogens are more electronegative than carbon. This means that the halogen atom becomes partially negatively charged and the carbon atom partially positively charged. We represent the partial charges using the delta symbol (δ), positioned above each atom:
The C-X bond polarity. Vaia Originals
Because of this polarity, halogenoalkanes experience permanent dipole-dipole forces between molecules. These are stronger than van der Waal forces and require more energy to overcome, influencing some of the physical properties of the molecules. Let's explore them now.
Take a look at Intermolecular Forces if you aren’t sure what the terms van der Waals forces and permanent dipole-dipole forces mean.
Melting and boiling points
Halogenoalkanes have higher boiling points than alkanes of similar chain length. This is due to two factors.
- Halogenoalkanes have a higher molecular mass than their respective alkanes. For example, methane (with a chain length of one carbon atom) has a molecular mass of 16.0, but chloromethane (also with a chain length of one carbon atom) has a molecular mass of 50.5. This means that chloromethane contains more electrons and can form larger temporary dipoles, and so experiences greater van der Waals forces between molecules.
- As explored above, C-X bonds are polar and contain a dipole moment. This means that halogenoalkanes additionally experience permanent dipole-dipole forces.
The increased strength of the intermolecular forces means more energy is required to separate the molecules. Halogenoalkanes, therefore, have higher boiling points than similar alkanes.
Remember that van der Waals forces are found between all molecules and are caused by temporary dipoles, whereas permanent dipole-dipole forces are only found between polar molecules with permanent dipoles.
Methane and chloromethane. Note chloromethane's permanent dipoles and higher molecular mass. Vaia Originals
In addition, longer halogenoalkanes have higher boiling points than shorter halogenoalkanes. They are larger molecules and experience greater van der Waals attraction. On the other hand, a branched chain hydrocarbon has a lower boiling point than a similar unbranched one. This is because the molecules cannot pack together as tightly, so the attraction between molecules is weaker.
For example, consider 1-chlorobutane and 1-chloromethylpropane. Both have the same molecular mass, but whilst the former is a straight-chain molecule, the latter is branched. This means that it can't pack together as closely - note how three 1-chlorobutane molecules can fit into the same space as just two 1-chloromethylpropane molecules.
Straight and branched halogenoalkanes. Vaia Originals
The boiling point of halogenoalkanes also varies depending on the halogen in the molecule. It is affected by two factors.
- As you move down the group in the periodic table, the halogen atoms increase in atomic mass. This means that they will experience stronger van der Waals forces.
- We saw above that as you move down the group, the halogen’s electronegativity decreases. This makes the C-X bond less polar and reduces the permanent dipole-dipole forces between molecules.
These two factors seem to oppose each other, so what is the trend in boiling points?
Well, melting and boiling points increase as you move down group 7 in the periodic table. The large increase in the number of electrons is therefore more important than the reduced permanent dipole strength. You can see this in the table below. Although chlorine has a greater electronegativity than iodine, 1-iodopropane has a higher boiling point than 1-chloropropane. This is because 1-iodopropane has more electrons and hence experiences stronger van der Waals attraction.
| Name | 1-chloropropane | 1-iodopropane |
| Electronegativity of halogen | 3.0 | 2.5 |
| Number of electrons | 42 | 78 |
| Boiling point (°C) | 46.6 | 102.6 |
Solubility
Halogenoalkanes are insoluble in water, despite their polarity. They are not polar enough to hydrogen bond with water molecules. However, they are highly soluble in organic solvents.
Production of halogenoalkanes
We can produce halogenoalkanes using a variety of different methods:
- Free radical substitution of alkanes, using Cl2 or Br2 in the presence of UV light.
- Electrophilic addition of alkenes using a halogen, X2, or hydrogen halide, HX.
- Substitution of alcohols, using various different reactants and conditions.
Don't worry - we cover these reactions in more detail in other parts of the course. Pay a visit to Chlorination to learn about free radical substitution, or head over to Reactions of Alkenes if you are curious about electrophilic addition reactions. You can also find out about alcohol substitution reactions in Reactions of Alcohol.
Reactivity of halogenoalkanes
Because of their polar C-X bond, halogenoalkanes are commonly attacked by nucleophiles.
A nucleophile is an electron pair donor.
Nucleophiles are negatively charged or partially negative charged molecules with at least one lone pair of electrons. They are attracted to positive, or partially positive, atoms such as the carbon in the C-X bond.
Common reactions involving halogenoalkanes include:
- Nucleophilic substitution with :OH- to form an alcohol.
- Nucleophilic substitution with ammonia to form an amine.
- Nucleophilic substitution with :CN- to form a nitrile.
- Elimination with :OH- to form an alkene.
Testing for halogenoalkanes
Got a solution of a halogenoalkane but not sure about its identity? Luckily for you, there's a simple test that we can use to find out which halogen is present. It involves another nucleophilic substitution reaction.
We first react the halogenoalkane with :OH- to form an alcohol and a halide ion. This is our nucleophilic substitution reaction. We then test for the halide ion using silver nitrate solution (AgNO3), acidified with dilute nitric acid (HNO3). The halide ion will react to form a coloured AgX precipitate, which tells you the halogen's identity. You can confirm your suspicions further by adding ammonia solution (NH3 (aq)). With any luck, you'll get the following results:
| Halogen present | Reaction with acidfied AgNO3 | Reaction with NH3(aq) | Further details |
| Fluorine | No observable reaction | | No precipitate as AgF is soluble in water |
| Chlorine | White precipitate | Dissolves in dilute NH3(aq) | |
| Bromine | Cream precipitate | Dissolves in concentrated NH3(aq) | |
| Iodine | Yellow precipitate | Insoluble in all concentrations of NH3(aq) | |
You could also omit the :OH- and instead add silver nitrate solution directly to the halogenoalkane. In this case, the water in the solution acts as the nucleophile. However, this reaction is a lot slower.
Factors affecting halogenoalkane reactivity
Two factors influence halogenoalkane reactivity:
Bond polarity
We learned earlier that the electronegativity of the halogen atom decreases as you move further down the group in the periodic table. This makes the C-X bond less polar. The carbon atom, now less positively charged, is less subject to attack by nucleophiles, so the bond is less reactive.
Bond strength
As you move further down group 7 in the periodic table, the C-X bond enthalpy decreases. This is because the halogen atom becomes larger and the shared pair of electrons is further from its nucleus. Thus, there is a weaker attraction between the electrons and the nucleus, making the bond easier to break and more reactive.
You might wonder which factor is more important. Well, experiments show that reactivity increases as you move down the group. This means that bond strength is a more important factor than bond polarity when it comes to reactivity.
A graph showing the relative strength of the C-X bond. Vaia Originals
Uses of halogenoalkanes
Finally, let's take a moment to consider some of the uses of halogenoalkanes.
- At the start of this article, we discussed how halogenoalkanes are useful components of many drugs, such as the antidepressant fluoxetine and anesthetic isoflurane.
- Halogenoalkanes are also used as solvents and fumigants.
- The non-stick coating Teflon is a polymer based on the halogenoalkane tetrafluoroethene.
- Chlorofluorocarbons (CFCs) rose to fame in the 20th century as popular features of many aerosols and refrigerants. However, CFCs are extremely dangerous due to their effect on the ozone layer. As a result, they are now banned in many countries. Alternatives include hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs).
You can explore how CFCs damage the ozone layer in the article Ozone Depletion.
Halogenoalkanes - Key takeaways
- Halogenoalkanes are alkanes where one or more hydrogen atoms have been substituted for a halogen atom, referred to as X. They are also known as haloalkanes or alkyl halides.
- Halogenoalkanes are named using standard nomenclature rules. They take the prefix fluoro-, chloro-, bromo- or iodo-.
- Halogenoalkanes are classified as primary, secondary, or tertiary.
- The C-X bond in halogenoalkanes is polar due to the differing electronegativities of the carbon and the halogen atoms. This means that they experience permanent dipole-dipole forces between molecules.
- Halogenoalkanes have higher boiling points than similar alkanes.
- We prepare halogenoalkanes from alkanes, alkenes or alcohols.
- Because of their polar bond, halogenoalkanes are susceptible to attack from nucleophiles.
- Halogenoalkanes become more reactive as you move further down the group in the periodic table.
- Uses of halogenoalkanes include many drugs, polymers, and solvents.
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