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Jetzt kostenlos anmeldenDid 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.
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.
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.
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. StudySmarter 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). StudySmarter 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. StudySmarter 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). StudySmarter 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.
List functional groups in alphabetical order.
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.
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. StudySmarter Originals
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. StudySmarter 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.
Halogenoalkanes have higher boiling points than alkanes of similar chain length. This is due to two factors.
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. StudySmarter 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. StudySmarter Originals
The boiling point of halogenoalkanes also varies depending on the halogen in the molecule. It is affected by two factors.
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 |
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:
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:
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.
Two factors influence halogenoalkane reactivity:
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.
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. StudySmarter Originals
Finally, let's take a moment to consider some of the uses of halogenoalkanes.
You can explore how CFCs damage the ozone layer in the article Ozone Depletion.
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.
Halogenoalkanes are organic compounds as they are based on a carbon chain.
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.
Yes - all halogenoalkanes are colourless.
What is a halogenoalkane?
An organic molecules formed from an alkane, where a halogen has replaced one or more hydrogen atoms.
State the prefix used to name halogenoalkanes with the following halogen atoms:
What is the difference between primary and secondary halogenoalkanes?
Primary halogenoalkanes have 0 or 1 R groups attached to the carbon atom the halogen is located on, whereas secondary halogenoalkanes have 2 R groups attached to the carbon.
Halogenoalkanes are _____ in water.
Insoluble
State the strongest type of intermolecular force found between halogenoalkane molecules.
Permanent dipole-dipole forces
Halogenoalkanes have _______ boiling points than alkanes of similar mass.
Higher
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