Chemistry · Ch 15 — Hydrocarbons
Isomerism in alkenes
Isomerism in alkenes
Alkenes with more than three carbon atoms show both structural isomerism and geometrical isomerism. Structural isomerism in alkenes includes both chain isomerism (the carbon skeleton itself differs) and position isomerism (the same carbon skeleton, but the C=C double bond sits at a different point along it) -- for the molecular formula C4H8 (butene), three distinct structures are possible: but-1-ene and but-2-ene, which share the same four-carbon chain but differ in where the double bond sits (making them position isomers of each other), and 2-methylprop-1-ene, which has a shorter three-carbon chain carrying a methyl branch (making it a chain isomer of the other two). Geometrical isomerism is a separate, additional kind of isomerism unique to compounds containing a C=C double bond (or a similarly restricted structural feature): because rotation around a double bond is not free (unlike the single-bond rotation of section 15.1.2), if the two atoms or groups attached to each end of the C=C are different from each other, the molecule can be locked into two distinct spatial arrangements that do not interconvert without breaking the pi bond. When the two identical or similar groups lie on the same side of the double bond, the arrangement is called the cis-isomer; when they lie on opposite sides, it is called the trans-isomer, and the two forms genuinely differ in physical properties such as melting point, boiling point and solubility -- cis-but-2-ene boils at 277 K while trans-but-2-ene boils at 274 K. Geometrical isomerism has firm limits, though: a terminal alkene (one carbon of the double bond is a plain =CH2, carrying two identical hydrogens) can never show cis-trans isomerism, and neither can a 1,1-disubstituted alkene (one carbon of the double bond carries two ident …
Worked out. C4H8 can be drawn three ways: (I) but-1-ene, H2C=CH-CH2-CH3, double bond at the end of a 4-carbon chain; (II) but-2-ene, H3C-CH=CH-CH3, double bond in the middle of the same 4-carbon chain; (III) 2-methylprop-1-ene, H2C=C(CH3)-CH3, a double bond on a shorter 3-carbon chain carrying a methyl branch. Structures I and III (and II and III) are chain isomers because their carbon-chain lengths differ; structures I and II are position isomers because they share the same 4-carbon chain but differ in where t …
Worked out. When the two atoms/groups on each end of a C=C double bond are different, two distinct spatial arrangements are possible: in the cis-isomer the two identical/similar groups lie on the same side of the double bond, and in the trans-isomer they lie on opposite sides; these differ measurably in physical properties. For but-2-ene, cis-but-2-ene (both CH3 groups on the same side, both H's on the same side) boils at 277 K, while trans-but-2-ene (the two CH3 groups on opposite sides) boils at 274 K. The chapter also notes three rules of thumb: a terminal alkene (C=CH2 unit) can never show cis-trans isomerism because one carbon carries two identical H's; a 1,1-disubstituted alkene (C=CR2 unit) likewise cannot; but alkenes of the general forms RCH=CHR, R1R2C=CR1R2, R1CH=CR1R2, R1CH=CR2R3, R1CH=CHR2 and R1R2C=CR3R4 (each doubly-bonded …