We see below that 3-methyl-3-hexanol can be synthesized from the reaction of 2-pentanone with ethylmagnesium bromide. There is a scheme of a chemical reaction, where CH3CCH2CH2CH3, with an O atom double-bonded to the second (from left to right) carbon (2-pentanone) reacts with CH3CH2MgBr (ethylmagnesium bromide). The arrow goes from the single bond between CH3CH2 and MgBr to the second C atom of 2-pentanone. The second arrow goes from the double bond between the second C atom and the O atom of 2-pentanone to the O atom. An intermediate is CH3CCH2CH2CH3, with a CH2CH3 group and an O minus MgBr plus group attached to the second carbon. In the presence of H3O plus the intermediate goes to CH3CCH2CH2CH3, with a CH2CH3 group and an OH group attached to the second carbon (3-methyl-3-hexanol, a tertiary alcohol). What other combinations of ketone and Grignard reagent could be used to prepare the same tertiary alcohol