Chem Final Review Chapter 4
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Explain a chain rxn steps.
initiation: generates a reactive intermediate
prop: reactive intermediates react with a stable molecule to form a product and another reactive intermediate, allowing the chain to continue until the supply of reactants is exhauseted or the reactive intermediate is destroyed
term: side reactions that destroy reactive intermediates and tend to slow or stop the reaction
Steps in free-rad halogenation.
Initiation: rads are formed
The overall rxn is simply the sum of the propagation steps
- Propagation: a radical reacts to generate another radical
- -1) a chlorine radical abstracts a hydrogen to generate an alkyl radical
- -2) the alkyl radical reacts with Cl2 to generate the product
When the equilibrium constant is so large that the reamaining amounts of hte reactions are close to zero at equilibrium, such a reaction is said to __.
If the energy levels of the products are lower in energy than the reactants, then the reaction is what? and the equation has a __.
go to completion
- energetically favored
- a negative value for deltaG
What value for deltaH and deltaS will be more ideal?
- dH: negative = exo
- dS: += favorable
What is bond dissociation energy?
the amount of enthalpy required to break a particular bond homolytically (each atom has one e, forming free radicals)
What does homolytic cleavage form? heterolytic?
Energy is __ when bonds are formed, and energy is __ to break bonds.
Therefore, bond-dissociation enthalpies are always __. The overall enthalpy change for a rxn is the __.
sum of hte dissiation enthalpies of hte bonds broken minus the sum of the dissociation enthalpies of the bonds formed
A __ is teh relationship between the concentrations of the reactants and the observed reaction rate. Each reaction has its own __, determined __ by changing the concentrations of the reactants and measuring the change in the rate.
Each reaxtion has its own characteristic rate constant, kr. Its value depends on the conditions of the rxns, especially hte temp. This temp dependence is expressed by the __.
The __ is the minimu kinetic energy the molecules must have to overcome the repulsions between their electrons clouds when they collide.
The __ represents the energy difference between the reactants and the __, the highest-energy state in a molecular collision that leads to rxn. In effect, the __ is the barrier that must be overcome for the rxn to take place. The value of Ea is always __, and its magnitude depends on the relative energy of the __. The term __ implies that this config is the transition between the reactants and products and the molecules can either go on to products or return to reactants.
- transition state x2
- TS x2
Unlike reactants and products, a TS is __ and __. It is not an __, becuase an __ is a species that can exist for some finite length of time, even if it is very short. An __ has at least som estability
- unstable and cannot be isolated
- intermediate x3
On reaction energy diagrams, what does the vertical axis say? What about the horizontal axis?
What is the max on the graph?
- vertical: total potential energy
- horizontal: reaction coordinate progress of the rxn, going from reactants on left to products on the right)
- the TS (as well as the RLS)
How can we calculate how reactive the hydrogens are?
by dividing the amount of product observed by the number of hydrogens that can be replaced to give that product
The more highly subbedthe carbon atom, the __.
less energy is required to form the free radical
What is the difference between reactions of chlorine nad bromine?
- Cl: energy dif. between primary and secondary are similar
- Br: energy dif. is different
In an exo reaction, the TS is __. In an endo rxn, hte TS is __.
- Exo: TS closer to reactants in energy and in structure
- Endo: TS is closer to products in energy and in sructure
related species that are closer in energy are also closer in structure. The structure of the TS resembles the structure of the closest stable species
Carbocations: species containing a carbon atom with a positive charge: no nonbonding electrons
Carbanions: species with a trivalent carbon atom that bears a negative charge; eight electrons around the carbon atom: electron rich and a strong nucleophile
uncharged reactive intermediates with a divalent carbon atom
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