Alkanes and Alkyl Groups

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. One student sees a three carbon branch on the middle of a five carbon chain, and names the alkane three propyl pentane. That sounds correct, right? But the answer key marks it wrong! Naming comes up often in JEE, so let's find out why.

0:20The parts of an IUPAC name

So, a name equals prefixes, plus the word root, plus the primary suffix, plus the secondary suffix, as on screen. Let us take them one by one, okay? First, the word root gives the number of carbons in the parent chain, from one to ten. Meth is one, eth two, prop three, but four, then pent, hex, hept, oct, non and dec. Clear? Next, the primary suffix tells us about the bonds. Ane means saturated, only single bonds; ene, a carbon carbon double bond, the two lines; and yne, a triple bond, the three lines. So tell me, what do we call the saturated chain of six carbons? Yes, exactly! Six carbons is hex, all single bonds is ane, so hex plus ane gives hexane. That part was really simple, right? Now the secondary suffix names the main functional group: ol, al, the suffix spelled o n e, or oic acid. And the prefixes name the substituents, with numbers called locants for their positions. With me?

1:30Rule 1: the longest chain

Rule one: the parent is the longest continuous carbon chain. In the drawing on the far left, each corner and line end is a carbon, the hydrogens are not drawn, and red numbers are locants. The other three drawings are for later lessons. Okay? Carbon one to carbon six makes six carbons. But start at the bottom end of the lower branch, go up to carbon three, then right to carbon six: that is also six! So which of these two chains is the parent? Here is the rule: when two chains are equally long, take the one with more substituents. The numbered chain has two, a methyl on carbon two and an ethyl on carbon three. The other has just one branch, so the numbered chain wins: a hexane. Its full name is written under the drawing, and rule three will explain that order. Got it?

2:23Rule 2: lowest locants

Rule two is about the lowest locants. Number from the end that gives the lower number at the first point of difference, not the lowest sum. Remember that, it matters later. Clear? Simple case first: C H three, C H with C H three in brackets, C H two, C H three. The small numbers written low are subscripts, the dashes are bonds, and a group in brackets is a branch on the carbon before it. The branch is on carbon two from the left, three from the right, so two methyl butane, not three methyl butane. With me? Next, C H three in brackets with subscript two means two C H three groups on that carbon, and with subscript three, three of them. The chain is five carbons. The curly brackets hold the locants: two, four, four from the left, or two, two, four from the right. So which set wins? Compare them term by term: the first terms are equal, two and two, but at the second term, two is less than four, as the less than sign shows. So two, two, four wins. The double arrow means therefore: the name is two, two, four trimethyl pentane. Okay?

3:37Rule 3: alphabetical order, and ties

Rule three: substituents go in alphabetical order. Take the hexane we just met, with a methyl on carbon two and an ethyl on carbon three. Ethyl starts with e and methyl with m, so three ethyl, two methyl hexane. Right? Now, multipliers like di, tri and tetra are ignored when you sort, and so are sec and tert. So ethyl comes before dimethyl, as the brackets show: e before m, not d. But iso, neo and cyclo are counted. Got it? Now this heptane has a methyl and an ethyl, C two H five in brackets. From the left, methyl is on three and ethyl on five; from the right, the other way round. It is a perfect tie! So which end do we number from? Here is the tie rule: when both directions give the same locants, the group first in the alphabet gets the lower number. Ethyl beats methyl, so the name is three ethyl, five methyl heptane. Clear?

4:36Branched groups: C-1 is attached; name in brackets

Now the common alkyl groups. The dash at the end of each formula is the bond to the main chain. As the heading says, the attached carbon is C one, and a branched group's name goes in brackets. Isopropyl joins through its middle carbon: a two carbon chain with a methyl on carbon one, so one methyl ethyl. Okay? Isobutyl joins through the C H two, which is carbon one. Carbon two carries a methyl, and carbon three ends the chain: two methyl propyl. Right? Sec butyl joins through the C H that carries the methyl, so that is carbon one. With the C H two and C H three it makes propyl, with a methyl on carbon one: one methyl propyl. Clear? Tert butyl has three methyls on the attached carbon. One methyl extends the chain to ethyl, and two sit on carbon one: one, one dimethyl ethyl. With me? And neopentyl is that tert butyl part joined through a C H two. Carbon one is the C H two, and carbon two carries two methyls: two, two dimethyl propyl. Common names on the left, systematic names on the right, okay?

5:44Example 1: why 3-propylpentane is really 3-ethylhexane

Now Example one, our puzzle from the start. One student names this molecule three propyl pentane. Correct the name. The middle C H carries a propyl branch, three carbons in brackets. Pause the video and try it first. Done? Okay, let's check. The student took the five carbons in the main line as the chain. But is that really the longest chain? No, it is not, and that is the mistake! Start at the left end, come to the middle carbon, and turn into the propyl group. Three plus three gives six carbons, so a hexane, not a pentane. Rule one was broken, right? What is left over? The C H two and C H three on the right, an ethyl group, on carbon three from the nearer end, not four. So the correct name is three ethyl hexane. Puzzle solved: the student never looked for the longest chain. Got it?

6:44Example 2: naming 2,7,8-trimethyldecane, the lowest sum trap

Now a harder one, JEE level. Name this alkane. C H two in brackets with a subscript four outside means four C H two groups in a row. And there are three methyl branches in brackets. Pause the video and try it first. Done? Okay, let's check. The C H three and the C H make two carbons, the four C H two groups make it six, and the last four groups make it ten. So a decane with three methyl groups. Okay? Now the locants. From the left, the methyls sit on carbons two, seven and eight. From the right, on three, four and nine. Clear? Now many students add them up. Two plus seven plus eight is seventeen, and three plus four plus nine is sixteen. Sixteen is smaller, so we number from the right, correct? No, no, wait, that is the trap! Rule two says compare term by term, never the sums. The first terms are two and three, and two is less than three, so we number from the left. Got it? So the answer is two, seven, eight trimethyl decane. The sum would have given you three, four, nine, and a wrong name. With me?

7:59Quick revision: longest chain, lowest locants, A to Z order

So, quick revision. First, the longest chain, and in a tie, more substituents. Second, lowest locants at the first point of difference, not the lowest sum. Third, alphabetical order, ignoring di, tri, sec and tert, and in a locant tie, the group first in the alphabet wins the lower number. Now try the practice problems on this page. All the best!