When the given compound is treated with Lithium diisopropylamide (LDA) in THF (tetrahydrofuran) at low temperature, it acts as a strong base and abstracts a proton from the compound, generating a nucleophilic intermediate, usually an enolate. This step is critical because the LDA is effective in creating a more reactive species that can participate in further reactions.
Next, the introduction of CH3CH2I (ethyl iodide) comes into play. The nucleophilic intermediate generated in the first step will attack the ethyl group of CH3CH2I through an SN2 mechanism. This results in the formation of a new carbon-carbon bond.
The overall effect of this two-step procedure is that a new alkyl group (ethyl) is added to the original compound, leading to the formation of the final product. In summary, after treating the original compound with LDA followed by CH3CH2I, you will obtain the compound with the ethyl group substituted or added at the position where deprotonation occurred.