0620 · 11.2Naming organic compounds

Bonding, formulae and name clues

0:00Methane, ethanol and ethanoic acid sound like unrelated names, but each contains clues about its structure. Organic names describe carbon chains and the groups attached to them. Once you understand those clues, you can work from a name to a drawing. You can also read a structure and build its name. We will connect three things throughout this video: the name, the arrangement of atoms and the family of compounds.

0:21The aim is not to memorise isolated pictures. Instead, you will learn a method for constructing them. That method also helps you recognise missing atoms and incorrect bonds. The essential starting point is bonding. In these molecules, carbon forms four bonds, oxygen forms two and hydrogen forms one. A double bond counts as two bonds when checking an atom. These rules explain how many hydrogen atoms belong around a carbon chain.

0:46A displayed formula shows every atom and every bond. A structural formula gives the arrangement more compactly, grouping hydrogen atoms with the carbon they belong to. A molecular formula only gives the total number of each type of atom. These formats contain different amounts of information, so they are not interchangeable. For ethanol, the molecular formula tells us there are two carbon atoms, six hydrogen atoms and one oxygen atom.

1:11Its structural formula shows how those atoms are grouped. The displayed version makes every connection visible. Notice that the hydrogen attached to oxygen is shown separately from those attached to carbon. Organic names have two especially useful parts. The beginning tells us how many carbon atoms are in the chain. Meth means one, eth means two, prop means three and but means four. The ending usually identifies the family, which depends on the characteristic bonding or functional group.

1:37An ending of ane identifies an alkane, while ene identifies an alkene. Names ending in ol belong to alcohols. Oic acid indicates a carboxylic acid. These endings describe structural differences, not simply different spellings, so we should connect each one to a drawing.

Alkanes, alkenes and functional groups

1:53Methane has one carbon atom and belongs to the alkane family. Four hydrogen atoms complete the four bonds around its carbon. Ethane contains two carbon atoms joined by a single bond. Each carbon then needs three hydrogen atoms because one of its four bonds already connects it to the other carbon. Ethene also contains two carbon atoms, but its ending tells us that it is an alkene. Draw a carbon to carbon double bond first.

2:17Each carbon now needs only two hydrogen atoms to complete its four bonds. The different hydrogen count follows from the bonding rather than being an unrelated fact to memorise. Ethanol contains two carbon atoms and an alcohol group. That group is an oxygen atom bonded to hydrogen and to a carbon atom. Ethanoic acid also contains two carbons, but its acid group has two oxygen atoms. One oxygen has a double bond to carbon, while the other connects that carbon to hydrogen.

2:43The acid group is called a carboxyl group. Its carbon atom belongs to the main chain, so it must be included when counting carbons. Ethanoic acid therefore contains two carbons altogether, not two plus an extra acid carbon. This is a common counting mistake that changes the name.

Drawing unbranched alkanes and alkenes

3:00This next part is for the Extended paper. We will apply the same reasoning to unbranched compounds containing up to four carbon atoms. Unbranched means the carbon chain has no carbon side branches. A chain drawn with a bend is still unbranched if its connections form one continuous sequence. The first four unbranched alkanes are methane, ethane, propane and butane. To draw one, make the required carbon chain using single bonds.

3:26Then add hydrogen atoms until every carbon has four bonds. Terminal carbons need three hydrogens, while internal carbons need two. A structural formula then lists each carbon with its hydrogen atoms, in order along the chain. Consider butane as a worked construction. The prefix but tells us to draw four carbon atoms, and the ending ane tells us to join them using single bonds. Add three hydrogens at each end and two around each middle carbon.

3:51This gives ten hydrogen atoms altogether, so the molecular formula is C₄H₁₀. Alkanes are hydrocarbons, meaning they contain hydrogen and carbon only. For these molecules, the hydrogen count is twice the carbon count plus two. Alkenes with one double bond have twice as many hydrogens as carbons. These patterns are useful checks, but the displayed structure explains why they work. The unbranched alkenes here are ethene, propene, but-1-ene and but-2-ene.

4:17Methene is not part of this series because a carbon to carbon double bond needs at least two carbon atoms. With four carbons, the double bond can occupy different positions. The name needs a number to distinguish those arrangements. For but-1-ene, the double bond begins at carbon one and connects it to carbon two. In but-2-ene, it connects carbon two to carbon three. Number from whichever end gives the double bond the smaller position number.

4:40A structure apparently called but-3-ene becomes but-1-ene when numbered from the other end. To draw but-2-ene, begin with four carbons in a row. Put the double bond between the two middle atoms. Each outer carbon needs three hydrogens, while each middle carbon needs one. There are eight hydrogens altogether, consistent with a four carbon alkene containing one double bond.

Naming alcohols and carboxylic acids

5:00Alcohol names use the ending ol, and the oxygen to hydrogen group is called a hydroxyl group. Methanol has one carbon, while ethanol has two. A three carbon chain can carry that group on its end carbon or its middle carbon. Those compounds are propan-1-ol and propan-2-ol. The number tells us which carbon carries the hydroxyl group. In the first, it is on carbon one; in the second, it is on carbon two.

5:24These molecules contain identical totals of each element, but their atoms are connected differently. A molecular formula alone cannot distinguish them. Butan-1-ol and butan-2-ol are two four carbon alcohols. Number from the end nearer the hydroxyl group. A position described as three becomes two when the chain is numbered from the opposite end. Similarly, an apparent position four becomes one. Suppose you are asked to draw butan-2-ol. First construct an unbranched chain of four carbons with single bonds.

5:52Attach an oxygen to carbon two, then attach a hydrogen to that oxygen. Finally, fill each carbon’s remaining bonds with hydrogen atoms, checking the attachment carbon especially carefully. The carboxylic acids are methanoic acid, ethanoic acid, propanoic acid and butanoic acid. Each contains the same carboxyl group at the end of its chain. Methanoic acid has only the carbon inside that group. A hydrogen atom occupies the bond where the longer acids have another carbon.

6:21For butanoic acid, count four carbons altogether, including the carboxyl carbon. Draw three carbons connected to that final carbon, then add its two oxygen atoms. One oxygen forms a double bond; the other has single bonds to carbon and hydrogen. Complete the remaining carbon bonds with hydrogen atoms. Names often identify a family immediately, but molecular formulae require more care. A formula containing only carbon and hydrogen can match the alkane or alkene patterns we have discussed.

6:48However, atom totals do not always reveal the functional group uniquely. When a displayed formula is available, inspect its bonds rather than guessing from the totals. For an alkene, look for a carbon to carbon double bond. An oxygen to hydrogen group attached to an ordinary chain carbon identifies an alcohol in this topic. If that oxygen is attached to a carbon also double bonded to another oxygen, you have a carboxylic acid.

7:13The surrounding connections matter, not merely the presence of oxygen.

Naming and drawing esters

7:17This next part is for the Extended paper. Esters have names built from two pieces because their structures contain parts from an alcohol and a carboxylic acid. The alcohol supplies the first part of the name, ending in yl. The acid supplies the second part, with oic acid changed to oate. Ethanol provides the ethyl part, and ethanoic acid provides ethanoate. Together, the name is ethyl ethanoate. Its characteristic connection contains a carbon double bonded to oxygen and single bonded to another oxygen.

7:46That second oxygen connects to a carbon chain instead of hydrogen. This distinction separates an ester from a carboxylic acid. The acid contains an oxygen to hydrogen bond within its carboxyl group. In the ester, that hydrogen is replaced by an alcohol-derived carbon group. There is no oxygen to hydrogen bond in these ester structures. The straight-chain alcohol-derived groups are methyl, ethyl, propyl and butyl. The acid-derived names are methanoate, ethanoate, propanoate and butanoate.

8:14Combining a column with a row gives the corresponding ester name. For example, methanol and propanoic acid give methyl propanoate. The carbon limit applies separately to the starting alcohol and acid. Butan-1-ol and butanoic acid each contain four carbons. Their ester can therefore contain eight carbons altogether. Do not incorrectly restrict the complete ester molecule to four carbons. Let us build methyl propanoate step by step. The acid-derived side needs three carbons, including the carbon double bonded to oxygen.

8:44Methyl requires one carbon on the other side of the linking oxygen. Add the hydrogen atoms last, then check four bonds around every carbon. Reading an ester backwards uses the same method. Locate the oxygen with two single bonds and inspect the carbon group on the side away from the carbon double bonded to oxygen. That gives the first word. Then count the acid-derived carbons, including the carbon double bonded to oxygen, to find the second word.

9:09The same construction works for the longest straight-chain example here, butyl butanoate. Draw four acid-derived carbons on one side and four alcohol-derived carbons on the other. Place the linking oxygen between those sections. Check that the acid-derived carbon next to it also has a double bond to oxygen. The esters you need are made from alcohols with the hydroxyl group on the end carbon. Propyl comes from propan-1-ol, and butyl comes from butan-1-ol.

9:34In each ester, the linking oxygen joins the end carbon of that straight chain. A common mistake is to name the acid part first, so always write the alcohol-derived word first.

Reaction products and naming recap

9:45Naming also applies to products encountered in the reactions of alkanes, alkenes, alcohols and carboxylic acids. We will focus only on recognising their names and structures here. The reaction conditions and explanations belong in those separate videos. Several products are molecules you can now construct without learning a new naming system. Replacing one hydrogen in methane with chlorine produces chloromethane. Adding bromine across ethene’s double bond produces 1,2-dibromoethane. Its carbon atoms now share a single bond, with one bromine attached to each.

10:17Adding hydrogen to ethene gives ethane, while adding steam gives ethanol. Complete combustion produces carbon dioxide and water. Hydrogen chloride is another product when methane reacts with chlorine. Cracking can produce smaller alkanes and alkenes, and sometimes hydrogen. Ethanol can be oxidised to ethanoic acid, while fermentation produces ethanol and carbon dioxide. Carboxylic acid reactions can also produce salts, water, hydrogen or carbon dioxide. Salts of ethanoic acid contain the ethanoate ion, giving names such as sodium ethanoate or magnesium ethanoate.

10:51These salts are ionic, so do not draw them as single covalent molecules. Ester formation gives the named ester and water. An exam question might ask you to state the type of compound and name this displayed structure. The oxygen to hydrogen group identifies an alcohol. Its unbranched chain contains four carbon atoms, giving the beginning but. Numbering from the nearer end places the hydroxyl group on carbon two, so its name is butan-2-ol.

11:16If asked to draw a displayed formula, show every atom and every bond. Leaving a group written as a compact structural formula does not meet that instruction. Check that double bonds have two lines and that each oxygen forms two bonds. These checks prevent missing hydrogens and impossible carbon atoms. The most useful naming method is to identify the family, count the carbon chain and locate any group needing a position number.

11:39For esters, separate the alcohol-derived part from the acid-derived part before naming either. Include the carboxyl carbon in the acid count. Build the bonds first and add hydrogen atoms afterwards. Organic names are therefore instructions for arranging atoms. The beginning gives the chain length, the ending identifies the family and a position number removes ambiguity. Displayed formulae reveal every connection, while molecular formulae give only atom totals. Understanding those relationships lets you move confidently between a name and its structure.