How do I solve this problem on Idomethane?

Question

Iodomethane (CH3I) is a commonly used reagent in organic chemistry. When used properly, this reagent allows chemists to introduce methyl groups in many different useful applications. The chemical does pose a risk as a carcinogen, possibly owing to iodomethane’s ability to react with portions of the DNA strand (of they were to come in contact). Consider the following hypothetical initial rates data:

[DNA]0 (mmol/L) [CH3I]0 (mmol/L) Initial Rate (mmol/L s)
0.1 0.1 3.20 × 10-4
0.1 0.2 6.40 × 10-4
0.2 0.2 1.28 × 10-3
Which of the following could be a possible mechanism to explain the initial rate data?

Mechanism I

DNA + CH3I -> DNA + CH3+ + I-

Mechanism II

CH3I -> DNA + CH3+ + I- Slow

DNA + CH3+ -> DNA-CH3+

Answer 1

From initial rate data, comparing experiment 1 and 2, it is seen that rate doubles as [CH3I] doubles.  Comparing experiments 2 and 3, it is seen that rate doubles as [DNA] doubles.  These data show that the rate is first order in both reactants, and the rate equation can be written as rate = k[DNA][CH3I].

 

Mechanism I would be consistent with the rate law, but not with the concept that CH3I interacts with DNA, because the DNA appears as a product in mechanism I indicating no change in the DNA. Excluding the biological implications, and only looking at the kinetics of the reaction, Mechanism I would be a possible mechanism.

 

Mechanism II is not consistent with the initial rate data.  The rate equation for the elementary rate limiting step in this proposed mechanism is rate = k[CHI] which is NOT the same as the actual rate equation derived from the data. Furthermore, DNA does not appear in the final reaction according to mechanism II.