Q.Find out the structures of two thyroid hormones T3 (triiodothyronine) and T4 (thyroxine). How do these help our body ?

Number the chain to locate the double bond and the halogen-bearing carbon: the Br sits one carbon away from the C=C, the defining position of an allylic halide.

CH₃–CH=CH–C(Br)(CH₃)₂: label the chain C1(CH₃)–C2H=C3H–C4(Br)(CH₃)₂. The double bond is between C2 and C3; the bromine is on C4, which is bonded to C3 (an alkene carbon) but is itself sp³ and NOT part of the double bond. A halogen on an sp³ carbon directly attached to a C=C carbon (but not itself part of the double bond) defines an allylic halide — distinct from a vinylic halide (X directly on an alkene carbon) …

A vinylic halide has X on the double-bonded ($sp^2$) carbon itself: $CH_2=CH-X$.

Halogen derivatives are classified by where the halogen atom is attached:

• Alkyl halide: X on an $sp^3$ carbon, e.g. option (a).
• Vinylic halide: X directly attached to an $sp^2$ carbon of a C=C double bond, e.g. $CH_2=CH-X$. …

The halogen sits directly on one of the alkene's own double-bonded carbons, not on an adjacent (allylic) carbon.

In a vinylic halide, the C–X bond is on a carbon that is itself part of the C=C double bond (sp² hybridized), e.g. CH₂=CHCl. This is distinct from an allylic halide, where the halogen is on an sp³ carbon adjacent to (but not part of) the double bond, e.g. CH₂=CH–CH₂Cl. Vinylic (and aryl) C–X bonds are much less reactive towards nucleophilic substitution than a normal alkyl C–X bond, …

An allylic halide has the halogen on an $sp^3$ carbon directly adjacent to a C=C double bond.

An allylic halide is one in which the halogen atom is attached to an $sp^3$-hybridised carbon that is directly bonded to a carbon of a carbon-carbon double bond (i.e. $X$ is on the carbon next to $C=C$). Of the options: a secondary alkyl halide with no adjacent double bond is a simple alkyl halide; $CH_2=CH-X$ has $X$ directly on the double-bond carbon (vinylic halide, not allylic …

A vinylic halide has the halogen on an sp$^2$ (C=C) carbon; 1,2-dichloroethene, ClCH=CHCl, has both Cl atoms directly on the double-bond carbons — option (b).

Vinylic halides are compounds in which the halogen atom is bonded directly to one of the carbon atoms of a C=C double bond (i.e. the halogen is on an sp$^2$-hybridised vinylic carbon), e.g. CH$_2$=CHCl (vinyl chloride). 1,2-Dichloroethene, Cl–CH=CH–Cl, has both chlorine atoms attached directly to the two alkene carbons, so it is a vinylic dihalide. Dichloroethane and ethylene chloride (which are actually the same compound, ClCH$_2$CH$_2$Cl) have the halogens on sp$^3$ (saturated) c …