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WHEN THE INSTRUCTOR ASKS FOR A ! MECHANISM ! PowerPoint PPT Presentation


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WHEN THE INSTRUCTOR ASKS FOR A ! MECHANISM !. DRAWING MECHANISMS. In this presentation we will address the question of how to present a mechanism when it is asked for in a problem or in an exam question. At the same time the points emphasized here should

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WHEN THE INSTRUCTOR ASKS FOR A ! MECHANISM !

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When the instructor asks for a mechanism

WHEN THE INSTRUCTOR ASKS FOR A

! MECHANISM !

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

DRAWING MECHANISMS

In this presentation we will address the question of

how to present a mechanism when it is asked for in

a problem or in an exam question.

At the same time the points emphasized here should

help you to know what to study when you are presented

with a mechanism in your reading assigments.

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

STEPS AND INTERMEDIATES

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

STEPS

A mechanism is a step-by-step specification

of what happens during the course of a reaction.

Your mechanism must have a step for the

formation of each intermediate formed during the

course of the reaction.

There are two styles of writing a mechanism,

the stepwise style and the merged-steps style.

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

.

.

STEPWISE MECHANISM

Instructors that use this style like each step on a separate

line.You must redraw each intermediate that is formed on

the next line before you can continue.

STEP 1

fast

..

..

..

..

+

+

:

:

STEP 2

slow

..

:

STEP 3

..

..

fast

:

:

:

..

..

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

MERGED STEPS MECHANISM

It is not necessary to have a line for each step in this

style. One step flows into another without redrawing

any intermediate.

fast

..

..

..

..

:

:

slow

..

:

:

..

..

..

:

:

..

fast

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

COMMON TO BOTH STYLES

Note that both styles use curved arrows.

All pairs of electrons are shown.

All formal charges are shown.

Also note that the rate-determining step (slow step)

must be indicated.

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

STEPWISE OR MERGED ?

Both methods of drawing a mechanism are valid.

You should determine which style of drawing

mechanisms your instructor prefers.

The line-by-line method is more formal and is often

used in papers dealing with the kinetics of reactions.

The merged style is more informal and is generally

used when drawing at a chalkboard or on paper.

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

ARROWS

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

ARROWS

Most instructors will require the use of curved arrows

to indicate which bonds are being broken or formed.

When using arrows, keep in mind the fundamental

definition of a curved arrow:

A curved arrow represents the movement of a

pair of electrons.

These arrows are never used to represent any other

type of movement.

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

TWO EXAMPLES

Correct

(WHAT THE ELECTRONS ARE DOING)

Electrons from base B: bond to H, which releases

its bonded pair to oxygen

Incorrect

(WHAT THE PROTON IS DOING)

An attempt to show that H+ is transferred to B:

incorrect use of an arrow

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

MORE ABOUT THE USE OF ARROWS

Each type of arrow used in organic chemistry has

a specific meaning; do not interchange their uses.

movement of a pair of electrons

Irreversible step (one way) arrow;

also used to indicate the products

of a reaction

Used to indicate a reversible reaction

step or an equilibrium

A variant of the equilibrium arrows above

Specifically used to designate resonance;

this arrow is never used to designate any

type of reaction or equilibrium.

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

DETAILS

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

ELECTRON PAIRS

You should show all the unshared pairs of electrons

in any region of the molecule where they are present

..... especiallyif that region is undergoing reaction or

change.

FORMAL CHARGES

Any atoms which carry a formal charge should be

clearly indicated by placing a +n or a -n sign

next to them.

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

RESONANCE

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

RESONANCE

Where resonance is important to the outcome of a step

it should be explicitly indicated using resonance

structures and resonance arrows ( ).

It is a good idea to collect the resonance-contributing

structures within a pair of braces (or brackets) to clearly

define them.

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

If you are using formal steps, you need only redraw the

most important contributing structure (the one that best

explains the formation of the next product).

Step 1

Step 2

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

PROTONS

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

PROTONS

“Free” protons ( H+ ) rarely exist in a solution. Protons

are usually bound to a solvent molecule or a a substrate.

While some instructors will allow you to represent

protons as H+ (fully understanding that such a species

does not exist in solution), others may require you to

show the explicit species which supplies the proton.

NOT

EXPLICIT

or

EXPLICIT

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

MORE ABOUT STEPS

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

REVERSIBLE STEPS

Steps which are reversible, or those which represent

an equilibrium, should use a pair of arrows, indicating

that the step may proceed both forward and backward.

When learning a mechanism, it is important to learn

which steps are reversible, and which are not. Steps

which are irreversible, are shown by using a single

unidirectional arrow.

- Most exchanges of protons are reversible and fast.

- Most rearrangement steps are irreversible.

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

..

:O-H

H

..

:O-H

H

SLOW STEPS

Most instructors would like you to label the

rate-determining-step with the word “slow”.

-

..

slow

H2O

:Cl:

..

R+

+

R-Cl:

..

1)

..

..

R+

R-

O-H

2)

+

H

..

..

..

3)

R-

O-H

R-

O:

H-

O-H

+

+

+

H

H

H

product

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

pH OF MEDIUM

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

pH OF THE MEDIUM

Keep the following rules in mind:

Anions are appropriate in basic solution.

Cations are appropriate in acidic solution.

Students often err by trying to invoke a strong base

(for example, hydroxide ion) in a solution like 3M H2SO4,

where the pH is considerably less than 7.0.

This situation has a very low probability!

Conversely, hydronium ions are not found in a basic

solution where the pH is considerably above 7.0.

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

KEEP THE pH OF THE MEDIUM IN MIND

Keep the pH of the medium (solvent) in mind when

you draw a mechanism.

The following solvolysis steps could take place in an

aqueous acid solution .....

all species are

neutral or they

are cations

..... but this step could not occur

there would be

insufficient OH-

ion in an acid

solution

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

WORDS

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

A MECHANISM IS NOT AN ESSAY

A mechanism is not an essay. It should consist of

equations, structures, and steps.

A picture is worth a thousand words if it is drawn

precisely and with care.

A mechanism is a series of pictures.

Brief marginal comments and identifying labels are

appropriate, but paragraphs are not.

Always be concise in your use of words.

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

STEREOCHEMISTRY

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

STEREOCHEMISTRY

In a step where the stereochemistry is important, it is a

good idea to redraw the structure in a three-dimensional

drawing showing its correct configuration or conformation.

-

..

:

:

-

..

..

:Br:

+

..

(R)

(S)

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

INDICATING STEREOCHEMISTRY

Stereochemistry should be clearly indicated using

the methods you learned earlier in the course.

In this structure, a student intended

the Br groups to be trans -

however, they could also be interpreted

to be cis.

NOT CLEAR

Use wedged and dashed lines to be clear!

...or use a view

from the edge :

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

SUMMARY

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

SUMMARY

If you pay careful attention to all of the points raised

in this presentation the next time you draw a mechanism,

you will surely have a better chance of getting it right,

and of getting more points for your answer.

A list of the common errors that a student may be

marked down for are listed on the next page.

Chem 211, Prof. Tarek Ghaddar


When the instructor asks for a mechanism

COMMON STUDENT ERRORS IN DRAWING A MECHANISM

using a pentavalent carbon (except for certain transition states)

incorrect (wrong way) use of curved arrows

missing curved arrow

missing formal charge

missing unshared electrons in a crucial step

failure to recognize resonance in an intermediate

incorrect use of resonance or equilibrium arrows

failure to recognize a reversible, or an irreversible, step

failure to label the “slow” or rate-determining step

incorrect species/medium recognition

using OH- in an acidic medium,etc.

using H+ in a basic medium, etc.

using a “free” proton when the actual donor species is important

missing a carbocation rearrangement

missing a reaction step, or combining two steps into one

failure to show an intermediate

ignoring the stereochemical outcome, or using structures which

do not unambiguously show the stereochemistry

not identifying products as meso, enantiomers, or diastereomers

Chem 211, Prof. Tarek Ghaddar


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