What is 'X' in the given reaction?

The reaction between ethylene glycol and oxalic acid at $$210^\circ \mathrm{C}$$ yields ethene as the major product.

Initially, the two hydroxyl groups of ethylene glycol undergo double esterification with the two carboxylic acid groups of oxalic acid. Two molecules of water are eliminated, forming a five-membered cyclic diester intermediate known as ethylene oxalate.

$$\mathrm{HO-CH_2-CH_2-OH + HOOC-COOH \xrightarrow[\ ]{210^\circ C} \text{Ethylene Oxalate} + 2H_2O}$$

Due to the high temperature condition, the cyclic intermediate becomes unstable and undergoes rapid thermal decarboxylation. During this process, the ring breaks apart and releases two molecules of carbon dioxide gas.

The remaining electrons rearrange to form a carbon-carbon double bond between the two carbon atoms of the original ethylene glycol chain, producing ethene as the final product.

$$\mathrm{\text{Ethylene Oxalate} \xrightarrow{\Delta} CH_2=CH_2 + 2CO_2}$$

Overall reaction:

$$\mathrm{HO-CH_2-CH_2-OH + HOOC-COOH \xrightarrow[\ ]{210^\circ C} CH_2=CH_2 + 2CO_2 + 2H_2O}$$