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I think the double bond inside the ring of vitamin A should have an (E) configuration. Am I wrong? I don't think I'm assigning the priority correctly.

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The endocyclic double bond of vitamin A has a (Z) configuration. Here’s why:

First, let’s determine priorities of the atoms connected on the top side of it:

  1. C

    1. C (ring)
    2. C (methyl 1)
    3. C (methyl 2)
  2. C

    1. C
    2. (C) (ghost atom; due to the double bond)
    3. H

So the ‘ring side’ — the atom on the left in the way it is drawn at the moment — has a higher priority than the side chain (C at (1.3) beats H at (2.3)).

What about the bottom side?

  1. C (ring)

    1. C
    2. H
    3. H
  2. C (methyl group)

    1. H
    2. H
    3. H

Here again the ‘ring side’ has a higher priority than the methyl group: (1.1) carbon beats (2.1) hydrogen.

Both left-hand sides have a higher priority than their corresponding right-hand side. Thus, the double bond is (Z) configured.

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  • $\begingroup$ We should just buy every student a copy of Elial and make them read it cover to cover. $\endgroup$
    – Zhe
    Oct 5 '16 at 21:52
  • $\begingroup$ @Zhe We should just make sure students learn and understand their syllabi’s materials … $\endgroup$
    – Jan
    Oct 5 '16 at 21:53
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There are two groups of compounds with Vitamin A, first is retinoids, also know as vitamin A and provitamin A, known as carotenoids which is a vitamin A precursors( which contain at least one molecule of retinol).

Naturally retinoids exist in the form of retinol, retinal and retionic acid and they are derived from animal sources. There are hundered of Carotenioids which are mainly synthesized by plants and only 50 of them can act as vitamin A precursors.

Technically speaking, large number of geometrical isomers are theoretically possible for retinoids and carotenoids but due to steric hinderence many mono-cis and poli-cis isomer are rarely occurred in nature. All-trans forms are predominant in nature and more stable, but only those cis isomers occur which offer less steric hinderence and thermodynamically stable.

enter image description here

5-cis-retinol geometrical isomer is rather unstable and do not occur in that configuration in nature.

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  • $\begingroup$ 1) Two of your molecules lost a double bond. 2) There is no cis-configured extracyclic double bound in your ‘13-cis-retinol’. 3) This does not answer the question. $\endgroup$
    – Jan
    Oct 5 '16 at 14:32
  • $\begingroup$ edited and notified correction has been made. This picture only posted to show that 13-cis isomer is possible and stable, $\endgroup$
    – Khan
    Oct 5 '16 at 14:46
  • $\begingroup$ The pictures still does not show 13-cis in spite of it claiming to do so. The question is still not answered. $\endgroup$
    – Jan
    Oct 5 '16 at 14:54

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