Consider the following bromination: (CH3)3CH + Br2 ^Δ⟶(CH3)3CBr + HBr. a. Calculate ΔH^∘ for thi

Consider the following bromination: (CH3)3CH + Br2...

Key Concepts

Bond Dissociation Energy

Bond dissociation energy is a measure of the strength of a chemical bond, quantifying the energy required to break the bond in a molecule. In thermochemical calculations, these values are used to estimate the overall energy change of a reaction by considering the energy needed to break bonds in the reactants and the energy released upon forming new bonds in the products.

Free Radical Mechanism

A free radical mechanism involves a sequence of steps initiated by the formation of reactive radicals, which then propagate further reactions. This mechanism typically includes three stages—initiation, propagation, and termination—each playing a critical role in sustaining the chain reaction and ultimately determining the product distribution.

Initiation Step

The initiation step is the phase in a radical mechanism where bonds are broken to generate radicals that can propagate the reaction. This usually involves homolytic cleavage of a bond, often induced by heat or light, resulting in the formation of highly reactive radical species that set the stage for the chain reaction.

Propagation Step

Propagation steps are the sequences of reactions in a chain mechanism that sustain and amplify the radical process. In each propagation step, a radical reacts with a stable molecule to form a new radical and a product. These steps are characterized by a balance between bond breaking and bond formation, and their energetics can be analyzed by comparing bond dissociation energies to assess the overall enthalpy change.

Termination Step

Termination steps occur when radicals combine to form a stable product, effectively ending the chain propagation. These steps remove reactive radicals from the system, and can involve the recombination of two radicals or a radical reacting with another species, thereby preventing further chain reactions.

Reaction Energy Diagram

A reaction energy diagram graphically represents the energy changes that occur during a chemical reaction. In the context of radical reactions, it details the energy levels of reactants, intermediates, transition states, and products. For each propagation step, the diagram illustrates the activation energy required to reach the transition state as well as the relative exothermic or endothermic nature of the step.

Transition State Structures

Transition state structures represent the highest-energy configurations along the reaction pathway, where reactants are in the process of being converted to products. They are critical in understanding the reaction rate and mechanism, as they indicate the points of maximum energy barrier that must be overcome for the reaction to proceed. In radical reactions, these transition states may involve partial bond breaking and bond formation, reflecting the dynamic changes occurring during the propagation steps.

Transcript

00:03 This is the answer to chapter 15, problem number 55 from the smith organic chemistry textbook.

00:11 And this problem asks us to consider this bromination reaction of turputane.

00:20 And we're asked quite a bit about this reaction.

00:25 And so for part a, we're asked to calculate the delta h -naught by using the bond dissociation energies in table 6 .2.

00:36 Okay, and so i've drawn the reaction out here.

00:40 So we need to look at what's happening.

00:43 So this is a ch bond broken.

00:50 This is a br -br bond broken.

00:58 And so from that table 6 .2, to break a ch bond is going to cost 381.

01:11 Kilojoules per mole to break a bromine bromine bond is going to cost 192 kilojoules per mole and so when we add those together the left side of this reaction is going to require 573 kilojoules per mole so then looking at the right side of this reaction these are bonds being formed so here we have a carbon bromine bond formed.

01:51 And so that is going to release 272 kilojoules per mole.

02:00 And here we have a hydrogen bromine bond formed.

02:07 And that is going to release 368 kilojoules per mole.

02:17 And so our total energy released on the right side is going to be negative 6004.

02:24 40 kilojoules per mole.

02:31 And when we add those values together, what we get is that the delta h0 for this reaction is going to be negative 67 kilojoules per mole.

02:51 Okay.

02:53 So this is an exothermic reaction.

02:56 Okay.

02:56 So negative 67 kilojoules per mole.

02:59 All right.

02:59 So that's part a.