Final answer:
The cation at m/z 57 in the mass spectrum of 2-methylpentane is most likely the isopropyl cation, formed by cleavage of the bond between the second and third carbon in the parent chain.
Step-by-step explanation:
The question pertains to the mass spectrum of 2-methylpentane and identification of the fragmentation pattern that leads to a cation with a mass-to-charge ratio (m/z) of 57.
2-Methylpentane has the structure CH3-CH(CH3)-CH2-CH2-CH3. The cation at m/z 57 is typically formed by the cleavage of a C-C bond that results in a cation containing three carbon atoms. Considering the structure of 2-methylpentane, one of the fragments produced to give the m/z 57 cation could be the isopropyl cation, (CH3)2CH+. This cation results from breaking the bond between the second and third carbon in the parent chain, leaving a three-carbon fragment with the positive charge on the central carbon.
This aligns with the common fragmentation patterns observed in mass spectrometry of alkanes where branched carbons tend to form more stable cations.