| ORGANIC CHEMISTRY I CHEM 2323 |
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| ORGANIC CHEMISTRY I CHEM 2323 |
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CHAPTER 5 - ALKYL HALIDES
HOMOLYTIC and HETEROLYTIC CHEMISTRY
RELATIVE RATES OF COMPETING REACTIONS
Chemical behavior is a matter of relative rates of competing reactions.
The competing reaction which occurs the fastest, predominates (makes more product than the other competing reactions).
The predominate reaction is usually the reaction which was the easiest mechanism.
STRUCTURE: The Functional Group
The atom or group of atoms that defines the structure of a particular family of organic compounds and, at the same time, determines their properties is called the functional groups.
Alkyl halides are classified the same as hydrogens. The class of carbon the halide is attached to determines the classification of the halide.
Alkyl halides (RX) have higher boiling points (BP) than alkanes with the same number of carbons.
As the molecular weight (MW) increases the boiling point (BP) increases.
As branching increases, BP decreases.
Alkyl halides are insoluble in water.
Alkyl halides are soluble in low polarity solvents.
1. From alcohols (most common method)
2. Halogenation of hydrocarbons (too many products)
The problem is that the product of one reaction can be the reactant of a second reaction, thus, polyhalogenation may occur.
3. Addition of hydrogen halide (HX) to alkenes.
At this point, do not worry about which side of the double band the halogen would attach to.
4. Addition of halogens to alkenes
5. Addition of halogen to alkynes
REACTIONS: NUCLEOPHILIC ALIPHATIC SUBSTITUTION
In a substitution reaction, one group is replaced by another.
Electron rich reagents that tend to attach the nucleus of carbon are called nucleophilic reagents. When the attack results in substitution, the reaction is called nucleophilic substitution.
1. Nucleophilic substitution
This reaction will be covered in detail later.
2. Dehydrohalogenation: elimination
discussed in detail later
3. Preparation of Grignard Reagent
NUCLEOPHILIC ALIPHATIC SUBSTITUTION
The components required for nucleophilic substitution are: substrate, nucleophile, and solvent. The substrate consists of two parts, alkyl group (R) and leaving group (X).
RATE OF REACTION: Effect of Concentration
The rate of a chemical reaction can be expressed by the product of three factors:
rate = collision frequency x energy of collision x probability of collision
The field of chemistry that deals with rates of reactions, and in particular, with the concentrations dependency of the rates on concentrations is called kinetics.
If the concentration is higher, it provides more targets for collisions.
KINETICS OF NUCLEOPHILIC ALIPHATIC SUBSTITUTION
Second order kinetics
We will use the reaction of RX and :Z- as an example.
rate = k [RX] [:Z-] (second order)
- k = rate constant for a given temperature and solvent.
- [RX] is the concentration of methyl bromide.
- [:Z-] is the concentration of hydroxide.
If we double the concentration of either reagent, the math shows the rate would also double. Therefore, the rate is dependent on the concentration of both reactants. We would call this a SN2 reaction (second order kinetics).
First order kinetics
If using the above reaction, we find that if we double the concentration of RX and the rate doubles, but upon changing concentration of Z, no rate change occurs, we call this a SN1 reaction (first order kinetics).
rate = k [RX] (first order)
Structure in regard to kinetics
The structure of the substrate RX (the classification of X) plays a role in the type of kinetics that will occur.
SN2 = CH3X > 1o > 2o > 3o
SN1 = 3o > 2o > 1o > CH3X
A 3o halogen would more likely be of 1st order kinetics (SN1), while a 1o halogen would most likely be of second order kinetics (SN2).
Remember that a mechanism is a step by step procedure on what occurs in a reaction. Since we have two different ways the RX can interact with the nucleophile, we have two different mechanisms.
SN2 REACTION: Mechanism and Kinetics
SN2 INVERSION OF CONFIGURATION
A reaction which yields a product whose configuration is opposite to that of a reactant is called inverted.
SN2 are completely stereochemically inverse.
The alkyl group affects the reactivity of the SN2 mechanism.
More alkyl groups attached to the substrate carbon, the more interference the nucleophile has getting to the substrate carbon.
SN1 REACTION: Mechanism and Kinetics
In a SN1 mechanism, the rate of reaction only depends on one of the reactants.
rate = k [RX] (1st order kinetics)
The rate of the overall reaction is determined by the slow breaking of the RX bond to form the carbocation (step 1). Once formed, the carbocation reacts rapidly to form the product RZ (step 2).
A single step whose rate determines the overall rate of a stepwise reaction is called a rate-determining step.
The rate-determining step is slow because it demands energy to occur (step 1). The reaction step of the carbocation and the nucleophile combining goes rapidly because it is an energy releasing step (step 2).
A carbocation is a group of atoms that contains a carbon atom bearing only six electrons.
Carbocations are classified as 1o, 2o, or 3o after the carbon bearing the positive charge.
The carbocation is an exceedingly reactive particle. The carbocation has one goal, to react to obtain eight electrons (octet).
Because the nucleophile can attach to either the top or bottom of the carbocation, there is a 50% chance there will be stereochemical inversion and 50% that the nucleophile will attach to the same side the X left.
RELATIVE STABILITIES OF CARBOCATIONS
3O carbocations contain less energy than 1o carbocations. The less energy, the more stable.
SN1 - Ease of formation of carbocations
The more stable the carbocation, the faster it is formed.
Therefore, since the 3o carbocation is the most stable, it will form the fastest.
If a neighboring carbon to the carbocation can better accommodate the positive charge, the charge may shift.
Therefore, rearrangement of the product may occur in a carbocation reaction.
SN2 |
SN1 |
2nd order kinetics |
1st order kinetics |
complete stereochemical inversion |
racemization |
absence of rearrangement |
rearrangement |
CH3X >1O > 2O > 3O |
3O > 2O > 1O > CH3X |
rate = k [RX] [:Z] |
rate = k [RX] |
favors strong nucleophiles |
favors weak nucleophiles |
Insoluble in cold H2SO4
Inert to Br2 in CCl4
Inert to KMnO4
Inert to chromic anhydride
