Q.Classify the following reaction as addition, elimination or substitution: CH3-CH2-Br + alc. KOH --heat--> CH2=CH2 + KBr + H2O.

[!TLDR] H and Br are removed from adjacent carbons and a C=C double bond is formed, so this is an elimination reaction. [!ANSWER …

In this reaction, ethyl bromide, $\text{CH}_3-\text{CH}_2-\text{Br}$, is treated with hot alcoholic potassium hydroxide. A hydrogen atom is removed from one carbon and the bromine atom is removed from the adjacent carbon, and a new carbon-carbon pi bond forms between those two carbons in their place, giving ethene, $\text{CH}_2=\text{CH}_2$, along with $\text{KBr}$ and $\text{H}_2\text{O}$ as by-products. Because two atoms are removed from adjacent carbons of the substrate and a multiple bond is created, rather than one atom simply being swapped for another, this is an elimination reaction -- specifically, the very common case of dehydrohalogenation (loss of a hydrogen halide unit). …

Check whether two atoms/groups are removed from adjacent carbons of the substrate with a new multiple bond forming in their place; if so, classify the reaction as elimination, …

Confusing this with substitution simply because a leaving group (bromide) departs; the decisive difference is that elimination also removes a second atom (here, a …