Organic Chemistry Chapter 8- Halogenation and Radical Reactions

Define free radical

a compound that contains unpaired electrons

A halogenation reaction is an addition, substitution, or elimination reaction?

substitution

Give the general reaction of halogenation, what can X2 be?

A carbon chain reacts with Cl2 or Br2 in light or triangle (idk what triangle is :'( sorry) which produces a carbon chain with one Cl or Br and the side product is H-Cl or H-Br

Why can Br and Cl work in halogenation but I and F cannot?

F is too reactive, I is very unreactive

Based on the reaction mechanism of halogenation, it can also be called what reaction?

a free radical chain reaction

molecules with unpaired electrons are usually not _ enough to survive

stable

Carbon free radicals are very unstable, but like secondary and tertiary carbocations, they can exist as….

reaction intermediates

Name the kinds of carbon free radicals and describe them

methyl radical: zero carbons (all hydrogen)
primary radical: one carbon
secondary radical: two carbons
tertiary radical: three carbons

Rank the stability of carbon free radicals (most to least stable)

Tertiary
Secondary
Primary
Methyl

Why are tertiary free radicals the most stable? (2 reasons)

1.) Hyperconjugation
2.) Inductive effect

Define bond dissociation energy

the energy required to homolytically cleave a bond

Bonds can be broken in what two ways? Explain them.

1.) Homolytic Cleavage: bond splits equally, giving one electron to each atom.
2.) Heterolytic cleavage: Bond splits unequally, giving one atom two electrons and the other zero.

For both homolytic and heterolytic cleavage, delta H is positive or negative?

Positive

Do shorter or longer bonds have higher bond dissociation energy? Explain.

Shorter bonds. They are harder to break and therefore require more energy.

Do larger or smaller atoms have higher bond dissociation energy? Explain.

Smaller atoms. Large atoms have longer bonds which are harder to break.

Our of tertiary, secondary, and primary carbons, which has the highest bond dissociation energy?

Primary

In a free radical chain reaction, the first step is always called

the initiation step

After the initiation step, the next steps in free radical chain reactions are called

propagation steps

In a free radical chain reaction mechanism, you must always regenerate…

the free radical

A free radical chain reaction ends at what step?

Termination step

How can you know what the initiation step is?

It is the first step and it has non-radical compounds that produce radicals

How can you know what the propagation steps are?

They consume a free radical and produce free radicals

How can you know what the termination step is?

Radicals are consumed and do NOT produce radicals.

In the first step of halogenation, blue light is shined onto the mixture for a brief moment. This achieves what?

Only a small percentage of Cl2 molecules are cleaved into radicals

Fill in the blanks to describe the mechanism of halogenation
Step 1 Initiation: is added which _ chlorine hetero/homolytically.

Step 2 Propagation 1: ____ electron on chlorine attacks ______. This bond breaks and ____ electron goes to . This produces a carbon with __ and .

Step 3 Propagation 2: electron on carbon attacks . The bond between _ and chlorine break, going to the . This produces a a carbon with and a _ ion.

Step 1 Initiation: LIGHT is added which CLEAVES to chlorine HETEROLYTICALLY.

Step 2 Propagation 1: ONE electron on chlorine attacks HYDROGEN. This bond breaks and ONE electron goes to CARBON. This produces a carbon with ONE ELECTRON and A H-CL.

Step 3 Propagation 2: ONE electron on carbon attacks CHLORINE. The bond between CHLORINE and chlorine break, going to the OTHER CHLORINE. This produces a a carbon with CHLORINE and a CHLORINE ion.

The first step of chain propagation is fast/slow and has low/high activation energy?

It is slow and has high activation energy

How do you calculate delta H?

Energy of bonds broken minus energy of bonds formed

A positive delta H is exothermic or endothermic? A negative Delta H?

A positive delta H is exothermic. A negative Delta H is endothermic

The second propagation step is fast/slow and has low/high activation energy

Fast, low activation energy

Explain how you know if a reaction is unimolecular or bimolecular (molecularity).

Look at reactants. If one reactant its unimolecular. If two reactants its bimolecular

Explain how you find total bond dissociation energy for chain propagation steps.

You solve them individually then add them together.

We are viewing all halogenation reactions as what?

Monohalogenation reactions

Explain how to calculate percentage of tertiaty/secondary/primary products if only two types are present. Use tertiary and primary in this example

Step 1: Count all tertiary and primary hydrogens.
Step 2: Multiply the tertiary hydrogen by its rate of abstraction. Multiply the primary hydrogen by its rate of abstraction.
Step 3: Add them.
Step 4: Divide both numbers from step 2 over the total.
Done :)

Fill in the blanks with chlorination and bromination: is very selective for tertiary positions, while _ is not very selective.

bromination is very selective for tertiary positions, while chlorination is not very selective.

Define Hammond's postulate

The structure of the transition state depends on if a reaction is endothermic or exothermic. If endothermic the transition state will look more like product , if exothermic it will look more like reactant.

How can you determine if a reaction is endothermic or exothermic?

Find Delta H, positive values are endothermic, negative values are exothermic

Define allylic carbon

carbon adjacent to a carbon-carbon double bond

Define vinylic carbon

carbon on double bond

Define allylic substitution

A reaction in which an atom or group of atoms are substituted for another atom or group of atoms at an allylic carbon

Which is stronger an allylic or vinylic carbon hydrogen bond?

A vinylic carbon-hydrogen bond

Rank carbocation stability and free radical stability (they're the same :)
Allylic
tertiary
secondary
primary
methyl
benzylic

Allylic = benzylic
tertiary
secondary
primary
methyl

Give two cons of high temperature reactions

1.) Its difficult to get a high temperature
2.) If there are other functional groups in a compound, they can be weakened at higher temperatures

Doing a reaction with Br2 at 350 degrees gives what result?

A Br on a allylic position and an HBr

What reagent can be used to get the same results as high temperatures without heat?

NBS (N-bromosuccinimide) in dichloromethane

Fill in the blanks to give the mechanism of allylic bromination using NBS
Step 1 chain initiation: The __ and ____ of NBS break homolytically.

Step 2 chain propagation 1: The single electron of attacks hydrogen, who's bond breaks and goes to . This produces an HBr and a carbon chain with a bond an _.

Step 3 chain propagation 2: _ comes in. The single electron attacks one of the bromine. The bond between the bromines break. This produces a ion with a single electron and a carbon chain with _.

Step 1 chain initiation: The NITROGEN and BROMINE of nbs break homolytically.

Step 2 chain propagation 1: The single electron of BROMINE attacks hydrogen, who's bond breaks and goes to THE ADJACENT CARBON. This produces an HBr and a carbon chain with a DOUBLE bond a SINGLE ELECTRON.

Step 3 chain propagation 2: BROMINE 2 comes in. The single electron attacks one of the bromine. The bond between the bromines break. This produces a BROMINE ion with a single electron and a carbon chain with A BROMINE.

Chain termination is the opposite or reverse of

initiation step

Give the three steps to finding the products of NBS if the allylic carbons are not equivalent

1.) Look at starting material and determine how many different allylic hydrogens it has
2.) Draw all of the allylic radicals that would form if allylic hydrogens were removed. Also draw any resonance structures of these
3.) Put a bromine at the position of the unpaired electron in each resonance structure. Watch out for identical products.

When is HBr markonikov and when is it anti-markonikov?

It is markonikov when radicals are not present
It is anit-markonikov when peroxides or other sources of radicals are present

Give the mechanism of free radical addition of HBr by filling in blanks
Step 1 Initiation: Carbon chain breaks __ with light

step 2: The lone electron attacks ____ of HBr. The bond breaks, giving the electron to Br.

Step 3: The lone electron of Br attacks. the tertiary/primary carbon of the double bond. The double bond donates an electron to _.

Step 4: The lone electron of the carbon chain attacks _ of HBr. The bond of HBr breaks, giving an electron to ____. This produces a carbon chain with a ____ and _, and a ___ ion with a _____.

Step 1 Initiation: Carbon chain breaks HOMOLYTICALLY with light.

step 2: The lone electron attacks THE HYDROGEN of HBr. The bond breaks, giving the electron to Br.

Step 3: The lone electron of Br attacks. the PRIMARY carbon of the double bond. The double bond donates an electron to THE ADJACENT CARBON.

Step 4: The lone electron of the carbon chain attacks THE HYDROGEN of HBr. The bond of HBr breaks, giving an electron to BROMINE. This produces a carbon chain with a HYDROGEN and BROMINE, and a BROMINE ion with a SINGLE ELECTRON.