0620 · 11.1Formulae, functional groups and terminology

Molecular and displayed formulae

0:00Two molecules can contain exactly the same numbers of atoms, yet have different structures. Both molecules here contain four carbon atoms and ten hydrogen atoms. However, their carbon atoms are connected differently. Understanding organic formulae means recognising both the ingredients of a molecule and how those ingredients fit together. In this video, we will connect molecular drawings, general formulae and chemical families. You will learn how particular groups of atoms help determine chemical properties.

0:26We will also distinguish saturated compounds from unsaturated ones. Finally, we will explain how different compounds can share the same molecular formula. The main starting idea is that atoms form particular numbers of covalent bonds in these molecules. Carbon forms four bonds, hydrogen forms one, and oxygen forms two. A single bond counts as one, while a double bond counts as two. These numbers let us check whether a drawing represents a possible molecule.

0:53Methane provides a simple example. Its molecular formula, CH₄, tells us that each molecule contains one carbon atom and four hydrogen atoms. The displayed formula shows something more. Each hydrogen connects to the central carbon through a single covalent bond, so carbon has its required four bonds. A displayed formula shows all the atoms and all the bonds in a molecule. Each line represents a bond between the atoms at its ends.

1:17Double bonds must appear as two lines, rather than one thick line. Leaving hydrogen atoms hidden would mean the drawing was not a complete displayed formula. Consider this displayed formula of ethanol. The first carbon connects to three hydrogens and the second carbon. That second carbon connects to two hydrogens, the first carbon and oxygen. Oxygen connects to the second carbon and one hydrogen, giving it two bonds. Counting the atoms gives two carbons, six hydrogens and one oxygen.

1:45Notice that the hydrogen attached to oxygen still belongs in the total. A molecular formula records those totals, but does not show the connections. The displayed formula therefore contains information that the molecular formula alone cannot provide. To build a displayed formula, start with the connections between the carbon atoms and any other non-hydrogen atoms. Then add hydrogen atoms until each carbon has four bonds. Check that oxygen has two bonds and every hydrogen has one.

2:11Finally, count all the atoms to confirm the required formula.

Functional groups and homologous series

2:15Some parts of an organic molecule are especially important for its chemical behaviour. A functional group is an atom or group of atoms that determine the chemical properties of a homologous series. Think of it as a characteristic chemical feature. The surrounding carbon chain can change while that important feature remains. For alkenes, the characteristic feature is a carbon to carbon double bond. Alcohols contain an oxygen to hydrogen group attached to carbon.

2:38In carboxylic acids, the feature is the carboxyl group shown here. That group includes a carbon to oxygen double bond and an oxygen to hydrogen bond attached to the same carbon. Recognise the whole carboxyl group, not just its oxygen to hydrogen part. An alcohol and a carboxylic acid both contain that connection, but their complete functional groups differ. The neighbouring carbon to oxygen double bond changes the chemical behaviour.

3:03Identifying a small fragment without checking its surroundings can therefore give the wrong family. A homologous series is a family of similar compounds with similar chemical properties due to the presence of the same functional group. Members have a shared chemical feature, even when their carbon chains have different lengths. Alcohols form one such family, while carboxylic acids form another. Their particular reactions belong in the separate videos about those families.

3:25Alkanes are also a homologous series. Their molecules contain only carbon and hydrogen, with single bonds throughout. All alkanes share this pattern of single bonds, so they react in similar ways. Do not invent an oxygen-containing group or a double bond for an alkane.

Using general formulae

3:40A general formula describes the atom pattern across a homologous series. The letter shown as the subscript stands for a variable number of carbon atoms. Substituting a particular value gives the formula of an individual member. However, the carboxylic acid formula needs special care because its written functional group contains an additional carbon. For alkanes, the hydrogen count is twice the carbon count, plus two. This pattern applies to the open-chain alkanes studied here.

4:06Each carbon forms four bonds, but connections between neighbouring carbons use some of those bonding positions. Hydrogen atoms fill the remaining positions around the chain. Propane has three carbon atoms in a chain. Each end carbon has three hydrogens, while the middle carbon has two. Three plus two plus three gives eight hydrogens. The general formula gives the same result because twice three, plus two, equals eight. The end carbons explain the extra two in the alkane formula.

4:33Each middle carbon carries two hydrogens in an unbranched chain. An end carbon has only one carbon neighbour, so it carries an additional hydrogen. There are two ends, giving two additional hydrogens overall. For the open-chain alkenes considered here, each molecule contains one carbon to carbon double bond. Their general formula gives twice as many hydrogens as carbons. Compared with the corresponding alkane, the double bond uses an extra bonding position on each of two carbons.

5:00Those positions cannot also carry hydrogen atoms. Ethane contains two carbons and six hydrogens. With the same two carbons, ethene carries only four hydrogens. Replacing the single carbon to carbon connection with a double bond removes two available hydrogen positions. That is why the alkene pattern has two fewer hydrogens than the corresponding alkane pattern. For alcohols, the general formula separates the oxygen to hydrogen group from the rest of the molecule.

5:26The carbon-containing part has twice the carbon count, plus one hydrogen. The final group contributes one oxygen and another hydrogen. Keeping that group visible makes the family feature easier to recognise. For an alcohol containing two carbons, the carbon-containing part therefore has five hydrogens. The oxygen to hydrogen group contributes a sixth hydrogen. This gives ethanol, with two carbons, six hydrogens and one oxygen overall. Forgetting that final hydrogen would produce an incorrect molecular formula.

5:54In the carboxylic acid formula, count the carbons outside the carboxyl group first. If that number is two, the attached carbon-containing part has five hydrogens. The carboxyl group adds one carbon, two oxygens and one hydrogen. The whole molecule therefore contains three carbons, six hydrogens and two oxygens. This example shows why the variable in the acid formula does not count every carbon in the molecule. One carbon is already written inside the carboxyl group.

6:21Reading only the first subscript would miss it. Always expand the meaning of the complete formula before counting atoms.

Saturation and structural formulae

6:28A saturated compound has molecules in which all carbon to carbon bonds are single bonds. In contrast, an unsaturated compound has molecules in which one or more carbon to carbon bonds are not single bonds. For the alkenes here, that means a carbon to carbon double bond. The distinction concerns bonds between carbon atoms specifically. A carbon to oxygen double bond does not make a compound unsaturated under this definition.

6:52The carboxylic acid drawings shown earlier have only single bonds between their carbons. They are therefore saturated despite containing a double bond to oxygen. Check the atoms at both ends of the bond before deciding. This next part is for the Extended paper. A structural formula is an unambiguous description of the way the atoms in a molecule are arranged. Unambiguous means that the connections are clear rather than open to different interpretations.

7:15Unlike a displayed formula, it can group atoms together instead of drawing every bond. The structural formula of ethene shows two carbon groups separated by a double bond. Each carbon carries two hydrogens. Ethanol is written as a group with three hydrogens, followed by one with two, followed by oxygen attached to hydrogen. These grouped descriptions still specify how the atoms connect. The structural formula of methyl ethanoate gives another example of compact notation.

7:41Its middle carbon is double-bonded to one oxygen and singly bonded to another. That second oxygen connects to the final carbon group. You only need to interpret this arrangement here; making and naming esters belongs elsewhere in the course. Writing the molecular formula alone is not an equivalent description. It tells us the total number of each type of atom, without specifying their arrangement. A structural formula must provide those connections clearly.

8:06Grouped notation works because the bonding rules and the order of the groups make the structure definite. The fuller description of a homologous series links five characteristics. Members have the same functional group and the same general formula. Successive members differ by a CH₂ unit, containing one carbon and two hydrogens. They display a trend in physical properties and share similar chemical properties. Compare ethane, propane and butane, containing respectively two, three and four carbon atoms.

8:34Their hydrogen counts are six, eight and ten. Each step adds one carbon and two hydrogens. This is the repeated unit that connects successive members of the series. Boiling point is a physical property, whereas chemical behaviour describes how substances react. Boiling points generally increase as molecules become larger within a series. Larger molecules generally experience stronger attractions between one another, requiring more energy to separate them. Similar chemical properties arise because every member has the same functional group.

9:04Similar does not mean identical in every respect. Members can have different boiling points while retaining the same general kinds of chemical behaviour. The changing carbon chain accounts for a physical trend. Their shared functional group explains why they remain part of one chemical family.

Structural isomers

9:18Structural isomers are compounds with the same molecular formula, but different structural formulae. They contain the same totals of each type of atom. However, the atoms connect in different arrangements. This is not the same relationship as successive members of a homologous series, whose molecular formulae differ. For a worked example, compare butane and methylpropane. First, count the atoms in butane. Its four carbon groups carry three, two, two and three hydrogens.

9:44This gives four carbons and ten hydrogens, with all four carbons connected in one continuous chain. In methylpropane, the central carbon connects to three other carbons and one hydrogen. Each surrounding carbon carries three hydrogens. Three groups of three hydrogens, plus the central hydrogen, gives ten. Both compounds therefore have the molecular formula C₄H₁₀. Next, compare their connections rather than their positions on the page. Butane has no carbon attached directly to three other carbons.

10:12Methylpropane has a central carbon with exactly that arrangement. The different connections establish that these compounds are structural isomers. Rotating a molecule or bending its drawing does not create a structural isomer. Those changes leave the connections between atoms unchanged. A bent drawing of butane still represents butane. To establish structural isomerism, show a genuinely different pattern of connections and confirm the same molecular formula. But-1-ene and but-2-ene illustrate another kind of structural difference.

10:41Both have four carbons connected in a chain. In the first, the double bond is at the end; in the second, it lies between the middle carbons. The position of that connection changes the structural formula. For but-1-ene, the hydrogen counts are three, two, one and two, giving eight. But-2-ene has three, one, one and three, which is also eight in total. Each therefore has the molecular formula C₄H₈. Their different structural formulae make them structural isomers.

Exam method and recap

11:10An exam question might show this molecule and ask you to state its homologous series. It could also ask you to explain whether it is saturated or unsaturated, and state its molecular formula. Start by identifying the characteristic bond. Then count every atom rather than guessing from the overall shape. The compound belongs to the alkene homologous series. It is unsaturated because its molecule contains a carbon to carbon double bond.

11:32It contains three carbon atoms and six hydrogen atoms. Its molecular formula is therefore C₃H₆. If asked to draw a displayed formula, include every atom and every bond. A grouped structural formula would not satisfy that instruction. For an isomer question, check equal atom totals before comparing the connections. Simply giving two members of the same series does not demonstrate structural isomerism. The central idea is that different formulae answer different questions.

12:00Molecular formulae give atom totals, displayed formulae show every bond, and structural formulae specify arrangements compactly. General formulae describe patterns across families. Functional groups connect those family patterns to chemical properties. Keep three final distinctions clear. Saturation depends on carbon to carbon bonds, not just the presence of any double bond. Successive family members differ by one carbon and two hydrogens. Structural isomers instead have identical molecular formulae, but different arrangements of their atoms.