Halogenoalkanes are a type of chemical compound used in many pharmaceutical drugs. Did you know that around 20% of all drugs contain fluorine? Fluorine is usually found in alkyl fluoride, which is an example of a halogenoalkane.
In this article, we'll break down everything you need to know about halogenoalkanes in organic chemistry. First, we'll define what halogenoalkanes are and then explain how to name them. We'll also cover how halogenoalkanes can be categorized as primary, secondary, or tertiary.
Next, we'll take a closer look at the properties of halogenoalkanes. We'll explore how they are produced and examine their reactivity, including their role in nucleophilic substitution reactions. Finally, we'll discuss the various uses for halogenoalkanes.
Halogenoalkanes are a type of organic molecule that are formed by replacing one or more hydrogen atoms in an alkane with a halogen atom. They are also known as haloalkanes or alkyl halides.
In simpler terms, halogenoalkanes are similar to alkanes, but they have a halogen atom (represented by X) instead of hydrogen atoms. Halogens are a group of elements in the periodic table, also known as group 17. They all have seven electrons in their outer shell and tend to be highly electronegative.
It's important to note that astatine, the largest halogen, is radioactive and short-lived, so it's not commonly used. Fluorine, on the other hand, is the smallest halogen and has many important applications in various fields.
The general molecular formula for a halogenoalkane with a single halogen atom is CnH2n+1X. Examples include chloroethane, C2H5Cl, and bromomethane, CH3Br.
Halogenoalkanes are named using standard nomenclature rules and the appropriate prefix, as shown in the table below.
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.
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.
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:
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.
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.
Halogenoalkanes are classified as primary, secondary, or tertiary based on the number of alkyl groups attached to the carbon atom bonded to the halogen (C-X).
In a primary halogenoalkane (1°), the carbon atom containing the halogen is bonded to zero or one alkyl group. In a secondary halogenoalkane (2°), the carbon atom containing the halogen is bonded to two alkyl groups. In a tertiary halogenoalkane (3°), the carbon atom containing the halogen is bonded to three alkyl groups.
To make it easier to understand, we'll use some examples. In the following molecules, the halogen atom is highlighted in blue and the alkyl groups (or R groups) are highlighted in red.
Understanding the classification of halogenoalkanes is important because it can affect their reactivity in chemical reactions.
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. 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:
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.
Halogenoalkanes have higher boiling points than alkanes of similar chain length for two reasons. Firstly, halogenoalkanes have a higher molecular mass than their respective alkanes. This means that halogenoalkanes can form larger temporary dipoles and experience greater van der Waals forces between molecules. Secondly, C-X bonds in halogenoalkanes are polar and contain a dipole moment, meaning they also experience permanent dipole-dipole forces.
The increased strength of the intermolecular forces in halogenoalkanes means that more energy is required to separate the molecules, resulting in higher boiling points than similar alkanes. It's important to note 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. Overall, the higher boiling points of halogenoalkanes compared to alkanes make them useful in certain industrial applications, such as solvents and refrigerants.
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.
The boiling point of halogenoalkanes varies depending on the halogen present in the molecule, and is affected by two factors: the strength of van der Waals forces, which increase with increasing atomic mass of the halogen, and the strength of permanent dipole-dipole forces, which decrease with increasing electronegativity of the halogen.
As you move down Group 7 in the periodic table, the halogen atoms increase in atomic mass, resulting in stronger van der Waals forces. At the same time, the halogens become less electronegative, making the C-X bond less polar and reducing the strength of permanent dipole-dipole forces between molecules. However, despite the decrease in permanent dipole-dipole forces, the increase in the number of electrons with increasing atomic mass is the more important factor, leading to an overall increase in melting and boiling points as you move down Group 7. As an example, 1-iodopropane has a higher boiling point than 1-chloropropane, even though chlorine has a higher electronegativity than iodine, because 1-iodopropane has more electrons and experiences stronger van der Waals forces.
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.
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.
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.
If you have a solution of a halogenoalkane and are unsure of its identity, you can use a simple test to determine which halogen is present. This test involves a nucleophilic substitution reaction, where the halogenoalkane is reacted with :OH- to form an alcohol and a halide ion. The halide ion is then tested using silver nitrate solution (AgNO3), acidified with dilute nitric acid (HNO3).
The halide ion will react with the silver nitrate solution to form a coloured AgX precipitate, which will indicate the identity of the halogen. For example, if the precipitate is white, it indicates the presence of chlorine, if it is cream, it indicates the presence of bromine, and if it is yellow, it indicates the presence of iodine.
To confirm your suspicions further, you can add ammonia solution (NH3 (aq)). If the precipitate dissolves in dilute NH3(aq), it indicates the presence of chlorine. If it dissolves in concentrated NH3(aq), it indicates the presence of bromine. However, if it is insoluble in all concentrations of NH3(aq), it indicates the presence of iodine.
Alternatively, you can omit the :OH- and directly add silver nitrate solution to the halogenoalkane. In this case, the water in the solution acts as the nucleophile. However, this reaction is slower than the previous method.
Two factors influence halogenoalkane reactivity:
Bond polarity.Bond strength.
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.
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.
Are halogens good leaving groups?
In general, halogens are good leaving groups. As you move down the group in the periodic table, their ability to act as a leaving group increases due to decreasing bond enthalpy.
Are halogenoalkanes inorganic?
Halogenoalkanes are organic compounds as they are based on a carbon chain.
Are halogenoalkanes reactive?
Halogenoalkanes are moderately reactive due to their polar C-X bond. This allows them to be attacked by nucleophiles. Their reactivity increases as you move down the group in the periodic table.
Are all halogenoalkanes colourless?
Yes - all halogenoalkanes are colourless.
Join Shiken For FREEJoin For FREE