Dehydration of 2-Methylcyclohexanol Lab Quiz Prep

What is in the bottom of the still vital?

The Drierite, phosphoric acid, and 2-methylcyclohexanol are contained in the bottom portion of the still.

Why should very little starting material collect in the Hickman?

We do not want to reach the boiling point of starting material which will them cause material to condense.

The formation of an alkene will be validated by

IR. The exact product composition will be determined the subsequent class by Gas Chromatography (GC)

In Brazil

ethanol (dehydration process) makes ethylene then made into plastic polyethylene. This means plastic is made from grain instead of petroelum like the rest of the world.

Give a reason why percent yield may be over 100%

contains impurities that cause its mass to be greater than it actually would be if the product was pure

In the alkene formation from the 2o carbocation, what is the kinetic and thermodynamic product?

the 1-methyl cyclohexene is the thermodynamic product because it is the most stable. The 3-methyl cyclohexene is the kinetic product because it is easier to remove either of the two hydrogens on the methylene carbon (-CH2-) than the hydrogen on the tertiary carbon (C-

Phosphoric acid acts as * It is not stoichiometric.

the catalyst for the reaction, so without it the reaction would happen very slow or not at all.

The product distribution can be under either kinetic or thermodynamic control. Typically the conditions in this lab favor thermodynamic

thermodynamic control instead.

A reasonable yield of this alkene product is

40-70 %.

The Drierite will absorb the water from the

85% phosphoric acid and absorb the water that is liberated during the dehydration.

LeChatelier's Principle

A change imposed on a system at equilibrium will result in a shift in the equilibrium to minimize the impact of that imposed event

Catalyst

A substance that changes the speed of a reaction, but is present in its original concentration at the end of the reaction. It is not stoichiometric.

Alkenes with the highest degree of alkyl substitution are the lowest energy and most stable. When a reaction is under thermodynamic control the most energetically favored product is formed preferentially. vii The stability of alkenes is directly tied to the degree of substitution.

Alkenes with the highest degree of alkyl substitution are the lowest energy and most stable. When a reaction is under thermodynamic control the most energetically favored product is formed preferentially. vii The stability of alkenes is directly tied to the degree of substitution.

What serves as a boiling stone replacement and fractionation column

The Drierite

Why should there be very little water collected in the Hickman ring?

It will feel as though the product is coming out and not enough product will be formed

What complications can arise when interpreting the product IR

Not enough solution to study, accuracy varies and wrong information presented.

Phosphoric acid protonates the alcohol group and turns the poor leaving group (-OH) into a very good leaving group(-OH2 + ). The departure of the leaving group forms a 2o carbocation. This is a slow step and the rate-determining step (RDS).

The carbocation can form two of the products with the elimination of an H on an adjacent carbon, 1-methyl cyclohexene, and 3-methyl cyclohexene. Alternatively, the 2o carbocation can undergo a rearrangement forming a more stable 3o carbocation. This carbocation can form two products methylenecyclohexane and 1-methylcyclohexene.

Can the thermotic and kinetic control product be the same?

The kinetic and thermodynamic product may be the same or different depending on the exact reaction energetics

Hoffmann Product

The less substituted alkene product.

Zaitsev's Product

The more substituted alkene product.

The dehydration of 2-methyl cyclohexanol begins with the reversible protonation of the -0H by the acid, transforming it from a poor leaving group to a good leaving group(H2O), which forms a 2 o carbocation in the rate-determining step.. then

This group leaves as a water molecule, forming a 2 o carbocation in the rate determining step

Thermodynamic Control

When the most stable product forms in the highest percent. The product with the lowest energy (most stable) forms in the largest yield.

Kinetic Control

When the product with the lowest activation energy, and hence the fastest rate forms in the highest percent. (less stable)

Dehydration Removes

an -OH from one carbon and an -H from an adjacent carbon and forming a new C=C and an H2O molecule.

Alcohols

are readily dehydrated to alkenes by using an acid catalyst

Give reasons why percent yield may be less than 100%

incomplete reaction and the reactants are not completely converted to products.

Alcohol dehydration is the transformation of acetaldol (2- hydroxy butanal)

into crotonaldehyde (2-butenal).

Crotonaldehyde

is used in the manufacture of sorbic acid which is a widely used food preservative.

2- methyl cyclohexanol reacts in presence of phosphoric acid which forms.

methylenecyclohexane (102) 3-methylcyclohexene (104) 1-methylcyclohexene (110) One molecule cannot form 3 molecules.* One mole of starting material can form a total of one mole of anyone or all three products*