When ethanol is dehydrated with concentrated H2SO4 at 413K, the major product formed is ethoxyethane (also known as diethyl ether).

Here is a step-by-step explanation of the reaction:

1. Acid-catalyzed dehydration: In the presence of concentrated H2SO4, ethanol undergoes acid-catalyzed dehydration, where a water molecule is eliminated from ethanol to form an alkene.

C2H5OH (ethanol) → C2H4 (ethene) + H2O

2. Ether formation: The ethene formed in the previous step can react further with another ethanol molecule to form ethoxyethane (diethyl ether) through an electrophilic addition reaction.

C2H4 (ethene) + C2H5OH (ethanol) → C4H10O (ethoxyethane or diethyl ether)

In this reaction, the double bond of ethene acts as an electrophile, and the lone pair on the oxygen atom of ethanol acts as a nucleophile. The oxygen atom of ethanol attacks the electrophilic carbon atom of ethene, resulting in the formation of a new carbon-oxygen bond and the elimination of a water molecule.

3. Side reactions: It is important to note that other side reactions can also occur during the dehydration of ethanol. For example, if excess ethanol is present, it can react with the ethoxyethane formed to produce higher ethers. Additionally, if the reaction conditions are harsh or if the reaction is carried out for a longer time, the ethoxyethane can undergo further dehydration to form ethene and water.

Overall, the major product formed when ethanol is dehydrated with concentrated H2SO4 at 413K is ethoxyethane (diethyl ether).