Some data on substituent effects for the reaction of...

Some data on substituent effects for the reaction of trifluoroacetanilides with methoxide ion in methanol and methanol-OD are given below. Calculate the isotope effect for each reactant. Plot the rate data against appropriate Hammett substituent constants. What facets of the data are in specific agreement with the normal addition-intermediate mechanism proceeding through a tetrahedral intermediate? What facets of the data suggest other complications? Propose a mechanism that is consistent with the data given.

Some data on substituent effects for the reaction of trifluoroacetanilides with
methoxide ion in methanol and methanol-OD are given below. Calculate the
isotope effect for each reactant. Plot the rate data against appropriate Hammett
substituent constants. What facets of the data are in specific agreement with
the normal addition-intermediate mechanism proceeding through a tetrahedral
intermediate? What facets of the data suggest other complications? Propose a
mechanism that is consistent with the data given.

Key Concepts

Hammett Equation

The Hammett equation relates the effect of substituents on the reactivity of aromatic compounds through substituent constants (sigma values). By correlating rate data with these constants, one can deduce how electronic effects influence the reaction mechanism, thereby providing a quantitative measure of the substituent impact on reaction rates and equilibria.

Isotope Effects

Isotope effects involve comparing the reaction rates of isotopically distinct compounds to gain insights into the reaction mechanism. In particular, differences arising from the substitution of hydrogen by deuterium (or other isotopes) can indicate which bonds are being broken or formed in the rate-determining step, thereby highlighting mechanistic details such as proton transfer events.

Tetrahedral Intermediate Mechanism

Many acyl substitution reactions proceed through a tetrahedral intermediate formed by the nucleophilic addition to a carbonyl carbon. This intermediate is a key species whose formation and subsequent collapse determine the reaction pathway. Data that align with predictions for such an intermediate support the conventional addition-elimination mechanism, whereas deviations may suggest alternative or additional steps.

Reaction Mechanism Analysis

This concept involves dissecting the overall reaction into individual steps and analyzing kinetic data, isotopic substitution studies, and substituent effects to build or revise the proposed mechanism. It requires combining experimental evidence with established theoretical frameworks to assess which steps are rate-determining and how molecular structure influences the reaction pathway.

Solvent Isotope Effects

The use of different isotopic variants of the solvent (for example, methanol versus methanol-OD) can provide valuable information about the involvement of solvent molecules in the reaction mechanism. Observations of rate variations when using isotope-substituted solvents help determine the role of solvent-mediated proton transfers and can signal the presence of additional mechanistic complications.

Transcript

00:01 So let's see if we can figure out some thermodynamic data for the reaction between methane and oxygen to make methanol.

00:10 First, let's try to figure out what the delta h and delta s of the reaction is.

00:17 And we're going to use the formula that the delta h of reaction is equal to sum of the number of moles times the enthalpy of formation of the products.

00:32 Minus the sum times the number of moles.

00:36 Here we're using m also for number of moles just to distinguish it from the product side times the enthalpy of formation of the reactants.

00:49 So to be able to do that, you have to look up the values for ch4 and ch30h.

00:55 We know that elements in their natural state, like oxygen here, has enthalpy of formation that is zero.

01:03 So when we're going to go solve this, we're going to go to the product side and get the value for the methanol.

01:11 And that is going to wind up b.

01:13 We have one mole.

01:16 And it has a delta h of formation of negative 201 .2 kilojoules per mole.

01:25 That's our only product.

01:28 So we're going to subtract that from our two reactants.

01:33 And we have one mole of ch4 with a heat of formation of negative 74 .8 kilojoules per mole.

01:47 And then we have our oxygen, but as mentioned before, elements in their natural state, have an enthalpy of formation of zero.

02:00 And when we do the math on this, we get the delta h to be negative, 126 .4 kilojoules.

02:09 We do the same thing for the enthalpy of reaction.

02:13 It's calculated the exact same way.

02:16 Just substitute an s in the equation up here for the h.

02:21 And look up the values of the standard entropy.

02:26 Now for entropy, elements in their natural state also have an entropy.

02:31 So you cannot assume that it's zero.

02:33 You must go and look it up.

02:37 And so we have our one mole and the standard entropy for methanol is 237 .6 joules per mole kelvin.

02:52 Only one reacting, but we have our two products.

02:56 So we have our one mole of ch4.

03:00 It has an enthalpy of 186 .3 joules per mole kelvin.

03:08 And we're going to add on to that our half mole of oxygen.

03:16 It has an enthalpy of 205 joules per mole kelvin.

03:23 And when we add that up, we see that the standard entropy here is negative 0 .0 .012 kilojoules per kelvin.

03:41 And that's dividing the answer our negative 51 .2 joules per kelvin by 1 ,000 to get it into kilojoules...

Please give Ace some feedback