2 quick module 3 questions

1. why does but-2-ene show geometrical isomerism, and but-1-ene not show it?

2. what is a primary, seconday and tertiary alcohol?

thanks

Reply 1

Prudy

1. But-1-ene doesn't because either cis or trans the molecule is the same H2C=C(H)C2H5
2. 1ary = c-c-c-c-OH
2ary = C-C(OH)-c

3ary = C
C-C-OH
C

DOes that help?

Reply 2

marcusfox

It's very easy to miss geometric isomers in exams if you take short-cuts in drawing the structural formulae. For example, it is very tempting to draw but-2-ene as

CH3CH=CHCH3

If you write it like this, you will almost certainly miss the fact that there are geometric isomers. If there is even the slightest hint in a question that isomers might be involved, always draw compounds containing carbon-carbon double bonds showing the correct bond angles (120°) around the carbon atoms at the ends of the bond. In other words, use the format shown in the last diagrams above.

To get geometric isomers you must have:

- Restricted rotation (often involving a carbon-carbon double bond for introductory purposes);

-Two different groups on the left-hand end of the bond and two different groups on the right-hand end. It doesn't matter whether the left-hand groups are the same as the right-hand ones or not.

Basically in but-2-ene both the groups on either side of the carbon atoms attached to the double bond can rotate. With but-1-ene, one end carbon atom is attached to the double bond and hence can't rotate.

In a primary (1°) alcohol, the carbon which carries the -OH group is only attached to one alkyl group.

An alkyl group is a group such as methyl, CH3, or ethyl, CH3CH2. These are groups containing chains of carbon atoms which may be branched.

Notice that it doesn't matter how complicated the attached alkyl group is. In each case there is only one linkage to an alkyl group from the CH2 group holding the -OH group.

There is an exception to this. Methanol, CH3OH, is counted as a primary alcohol even though there are no alkyl groups attached to the carbon with the -OH group on it.

Secondary alcohols

In a secondary (2°) alcohol, the carbon with the -OH group attached is joined directly to two alkyl groups, which may be the same or different.

Tertiary alcohols

In a tertiary (3°) alcohol, the carbon atom holding the -OH group is attached directly to three alkyl groups, which may be any combination of same or different.

Marcus