Classification, Functional Groups and Homologous Series

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Here is a mystery alkane. Its molar mass is eighty six grams per mole, and it has only primary and tertiary carbons. Can you name it? Sounds like guesswork, right? It is not. By the end of this video you will name it, using the general formula of a family and the classes of carbon atoms. Questions like this are good practice for the JEE exam. So, let's begin.

0:28Sorting by the carbon skeleton

So, how do we sort organic compounds? First, by the carbon skeleton. If the carbons form an open chain, straight or branched, the compound is acyclic, also called aliphatic, like butane or acetic acid. Okay? If the carbons close into a ring that is not aromatic, the compound is alicyclic, like cyclohexane and cyclohexene. Yes, cyclohexene has a double bond, but it is still not aromatic. Right? Then the aromatic compounds. Benzenoid ones contain a benzene ring, like benzene, naphthalene and aniline. Tropone and azulene are aromatic with no benzene ring, so they are non-benzenoid. Clear? Finally, heterocyclic compounds: rings that contain an atom other than carbon, such as oxygen, nitrogen or sulphur, the O, N and S on screen. Let me ask you this. Tetrahydrofuran has an oxygen in a saturated ring. Is it aromatic? No, it is not. Being a saturated ring, tetrahydrofuran, like piperidine, is an alicyclic heterocycle. But furan, thiophene and pyridine are aromatic heterocycles. So for any ring, check two things: an atom other than carbon, and aromaticity. Got it?

1:44Functional groups and homologous series

Now, the second big idea. The functional group is an atom, or a group of atoms, that gives a compound its characteristic chemical properties, like O H, CHO and COOH on screen. The small dash in front is the bond to the rest of the molecule. Okay? Now, compounds with the same functional group and the same general formula form one family, called a homologous series. With me? Look at methanol, ethanol and propanol on screen. The small numbers written low are subscripts, counting the atoms. Each member is the one before it, plus one C H two group. So tell me, by how many mass units does each member grow? Yes, exactly, it is fourteen! Carbon is twelve and two hydrogens are two, so each C H two adds fourteen u, where u is the atomic mass unit. Right? And members of a series have similar chemical properties, because they share the functional group, while their physical properties change gradually. Clear?

2:44General formulae of the families

Each series has a general formula, where small n is the number of carbon atoms. Alkanes are C n H two n plus two; the subscript two n plus two counts the hydrogens. And alkenes are C n H two n, okay? Alkynes and dienes are C n H two n minus two. And arenes, the benzene series, are C n H two n minus six. Right? Now with one oxygen: alcohols and ethers are C n H two n plus two O, while aldehydes and ketones are C n H two n O, okay? Acids and esters are C n H two n O two, and saturated amines are C n H two n plus three N, with one nitrogen. So, a quick test. If a compound fits C n H two n, is it surely an alkene? No, and this is a common trap. Cyclohexane, C six H twelve, has no double bond, yet it fits C n H two n. Look, several formulas on screen are shared by two families, like alcohols and ethers, so the formula alone may not fix the family. Clear?

3:47Primary, secondary, tertiary, quaternary

Now the last idea, and our puzzle needs it. Each carbon is classed by its carbon neighbours: one makes it primary, two secondary, three tertiary, and four quaternary. On screen, each number carries a small raised circle, like a degree sign. Okay? And each hydrogen takes the class of the carbon that carries it, so a hydrogen on a primary carbon is primary. Right? For alcohols and halides, look at the carbon carrying the O H group or the halogen, written X on screen. That carbon decides the class. Clear? But amines are different. An amine is classified by the number of carbons attached to the nitrogen. Remember this difference. With me? Now look at this halide. The bracket with a small two outside means two C H three groups, both on one C H that carries a bromine. That carbon has two carbon neighbours, so this is a secondary halide. Now swap the bromine for N H two, as in the badge. So it is a secondary amine, right? Oh no, careful, that is the trap! For amines we count the carbons on the nitrogen, and here the nitrogen carries just one carbon. So it is a primary amine, even though its carbon is secondary. Got it?

4:58Example 1: formula and molar mass of the fourth alkyne

Alright, now Example one. The fourth member of the homologous series of alkynes: what is its molecular formula, and what is its molar mass? Pause the video and try it first. Done? Okay, let's check. Alkynes are C n H two n minus two. Fourth member, so n equals four, giving C four H six, right? No, no, wait, that is the trap! The series starts at ethyne, C two H two, so the first member has n equal to two. Count on: two, three, four, five. The fourth member has n equal to five. Okay? Now put n equal to five in the formula. Two times five, minus two, gives eight hydrogens. So the fourth alkyne is C five H eight. Right? Now the molar mass, written capital M on screen. Five times twelve for carbon, plus eight times one for hydrogen, gives sixty plus eight, which is sixty eight. The unit, g mol to the power minus one, is read as grams per mole. So, C five H eight, and sixty eight grams per mole. Clear?

6:01Example 2: mass 86 alkane, only primary and tertiary carbons

Now, remember our puzzle from the start? Let's finish it. An alkane of molar mass eighty six grams per mole has only primary and tertiary carbons. Identify it. Pause the video and try it first. Done? Okay, let's check. Alkanes are C n H two n plus two. The carbons give twelve times n, and the hydrogens give two n plus two. So the molar mass is fourteen times n, plus two. Okay? Set fourteen times n plus two equal to eighty six. Fourteen times n is eighty four, so n is six. The double arrow on screen means therefore. So our alkane is C six H fourteen. Now the second clue. Which kinds of carbon are not allowed here? Yes, exactly, secondary and quaternary carbons are not allowed! In an alkane, a C H two group has two carbon neighbours, so it is secondary. And a carbon with no hydrogen has four carbon neighbours, so it is quaternary. So neither can appear in our molecule. Clear? So only C H three groups, which are primary, and C H groups, which are tertiary. To get six carbons and fourteen hydrogens, we need four C H three groups and two C H groups. Let us check: four times three is twelve, plus two, gives fourteen. With me? Now join them. The two C H groups bond to each other, and each carries two methyl groups. That is two, three dimethyl butane, a four carbon chain with methyl groups on carbons two and three. And that solves our puzzle. Got it?

7:30Quick revision: skeletons, homologous series, carbon classes

So, quick revision. By the skeleton, a compound is acyclic, alicyclic, aromatic or heterocyclic. Members of a homologous series share one functional group and differ by C H two, fourteen u. Primary to quaternary means counting carbon neighbours, but for amines, count the carbons on the nitrogen. Now try the practice problems on this page. All the best!