0620 · 11.4Alkanes

Alkane structure and reactivity

0:00Methane can burn as a fuel, yet it belongs to a family described as generally unreactive. Those facts are not contradictory. A substance can resist many reactions but still react under particular conditions. We will connect the structure of alkanes to their behaviour, then follow how chlorine replaces a hydrogen atom. A covalent bond is a shared pair of electrons between atoms. In the structures here, carbon forms four bonds, while hydrogen forms one.

0:25Each line in a displayed formula represents a bond. Counting those lines helps us understand the molecule and check whether a proposed product makes chemical sense. Alkanes are hydrocarbons, meaning compounds containing hydrogen and carbon only. Their bonding is single covalent only. Methane is the simplest example, with one carbon atom joined to four hydrogen atoms. There are no double or triple bonds anywhere in its structure. Ethane has two carbon atoms joined together, with three hydrogen atoms attached to each.

0:54Every connection is still a single covalent bond. Each carbon has four bonds altogether, including its connection to the neighbouring carbon. Changing the length of the carbon chain does not change this defining feature of an alkane. Alkanes are saturated hydrocarbons because their carbon to carbon bonds are single only. For a given carbon skeleton, they contain the maximum possible number of hydrogen atoms. The carbon atoms already have their full bonding capacity occupied.

1:19Adding another hydrogen would require changing the existing structure rather than simply attaching it. Saturated does not mean soaked in a liquid, and it does not mean completely unable to react. Here, it describes the bonding within a molecule. Methane is also saturated, even though it has no carbon to carbon connection. All its bonds are single, and its carbon already forms four bonds. Alkanes are generally unreactive because their carbon to carbon and carbon to hydrogen bonds are strong.

1:46Starting a reaction requires energy to break existing bonds. Under ordinary conditions, many substances do not readily react with them. Generally unreactive therefore means resistant to many reactions, not chemically inactive in every situation. The two reactions we need here are combustion and substitution by chlorine. Combustion involves an alkane reacting with oxygen and releasing energy. Chlorine substitution changes which atom is attached to carbon. Both reactions require suitable conditions, so neither contradicts the description of alkanes as generally unreactive.

Complete combustion of methane

2:17With sufficient oxygen, methane undergoes complete combustion. The word equation is methane plus oxygen gives carbon dioxide plus water. Carbon atoms end up in carbon dioxide, while hydrogen atoms end up in water. The reaction releases energy overall, but an initial energy input is needed to start it. The balanced symbol equation is CH₄ plus 2O₂ gives CO₂ plus 2H₂O. One carbon atom on each side accounts for the carbon dioxide.

2:44Four hydrogen atoms require two water molecules. Those products contain four oxygen atoms altogether, supplied by two oxygen molecules. An ignition source provides the initial energy needed for combustion. After burning begins, the released energy can help the reaction continue. This explains how a generally unreactive substance can still be useful as a fuel. Fuels are covered in their own video, and incomplete combustion appears in the video on air quality.

Chlorine substitution in alkanes

3:10This next part is for the Extended paper. In a substitution reaction, one atom or group of atoms is replaced by another atom or group of atoms. For an alkane reacting with chlorine, a hydrogen atom is replaced by a chlorine atom. The carbon framework remains in place. Imagine replacing one part of a model without rebuilding the whole thing. In methane, remove one hydrogen from its connection to carbon and put chlorine in that position.

3:34Carbon still forms four single bonds. The product is chloromethane, containing three hydrogen atoms and one chlorine atom attached to carbon. The replacement description tells us how the organic molecule changes, but it does not account for every atom involved. Chlorine enters the reaction as molecules containing two atoms. One becomes attached to carbon, while the other combines with the displaced hydrogen. This produces hydrogen chloride as a second product.

4:00The word equation is methane plus chlorine gives chloromethane plus hydrogen chloride. The balanced symbol equation is on the board. One methane molecule reacts with one chlorine molecule to give one molecule of each product. All the original atoms are still present, but some now have different bonding partners. This is a photochemical reaction, meaning a chemical reaction brought about by light. Ultraviolet light provides the activation energy. That is the minimum energy needed to start the reaction.

4:28Here, light supplies energy that can break the chlorine to chlorine bond, allowing the substitution process to begin. Ultraviolet light is a condition, not a reactant that becomes part of the products. That is why it is written above the reaction arrow. It should not be described as a catalyst. For this reaction, the important explanation is that ultraviolet light provides the activation energy. Our products are limited to monosubstitution, meaning replacement of one hydrogen atom in each alkane molecule.

4:56Methane therefore becomes chloromethane, not a molecule with several chlorine atoms attached. Other reactions can occur beyond this first replacement, but they are outside this lesson. Keep your product focused on exactly one substitution. Let us work through a different alkane rather than copy the methane example. Suppose ethane reacts with chlorine in ultraviolet light. We are asked to draw the displayed formulae of the products of monosubstitution. Start from ethane and check that both carbon atoms have four bonds.

5:23Choose the hydrogen at the right-hand end and replace it with chlorine. Keep the carbon to carbon single bond and every other hydrogen in place. The organic product is chloroethane. The hydrogen that was replaced combines with the other chlorine atom, so hydrogen chloride must also be shown. Check the drawing by counting bonds and atoms. Each carbon still has four bonds, while every hydrogen and chlorine has one. Across the products there are two carbon atoms, six hydrogen atoms and two chlorine atoms.

5:50Those totals match one ethane molecule and one chlorine molecule before the reaction. Several common exam mistakes come from treating substitution as addition. Attaching chlorine without removing hydrogen would give carbon five bonds in these examples. Replacing every hydrogen would ignore the instruction for monosubstitution. Drawing only the organic product would also miss hydrogen chloride and leave some atoms unaccounted for. A question might ask you to state why methane is a saturated hydrocarbon.

6:18A precise answer is that it contains hydrogen and carbon only, and has single covalent bonds only. If asked to describe its reaction with chlorine, name chloromethane and hydrogen chloride. State that ultraviolet light is required and provides the activation energy. The key connection is between structure and reaction conditions. Alkanes are saturated hydrocarbons with single covalent bonds only, and they are generally unreactive. They nevertheless undergo combustion and substitution by chlorine.

6:47In monosubstitution, one hydrogen is replaced by chlorine, with hydrogen chloride also formed and ultraviolet light providing the activation energy.