o chem exam 3

Practice problems/ Study guide for Exam 3: Major concepts per topic

Energy and organic chemistry:

• Draw a 2-step energy diagram, and be able to label all of the appropriate parts such as the

reactants, products, intermediate, transition states, rate limiting step, activation energy,

and whether the reaction is overall endothermic or exothermic.

• Draw a 1-step energy diagram and label all appropriate parts as mentioned above.

• Determine if the energy diagram given represents an E1, E2, SN1 or SN2 reaction, or a

combination of the mechanism choices.

• Rank carbocations, or carbanions in order of increasing stability.

• Terms: endothermic, exothermic, carbocation, carbanion, transition state, intermediate,

activation energy.

Chirality and direction of compounds:

• Be able to identify chiral carbons on a compound. Given the number of chiral centers,

calculate the number of stereoisomers possible for that compound (2n rule, where n = the

number of chiral carbons)

Mechanisms of reactions:

• SN2, SN1, E1, E2 mechanisms are the focus. Be able to determine the mechanism by

which the reactants converted to products, using the typical conditions for each type of

reaction.

• SN2 characteristics include the following: Primary or methyl substrate; strong

nucleophile; polar aprotic solvent; inversion at chiral carbon; no rearrangements

• SN1 characteristics include the following: secondary or tertiary substrate; weak

nucleophile; polar protic solvent; inversion and retention at chiral carbon; rearrangements

possible

• E1 characteristics: same as SN1. E1 products are usually found along with SN1 products,

and vice versa.

• E2 characteristics: secondary or tertiary substrate; strong nucleophile; solvent doesn’t

matter; always have an alkene as the product. **If the nucleophile was big (15 atoms or

bigger) then the least substituted alkene will be the major product. If nucleophile is

small, then the most substituted alkene will be the major product (Follows Zaitsev’s rule).

• Zaitsev’s Rule: The most stable alkene formed will have the most substituent groups

bonding to the alkene carbons.

• Terms: Lewis Acid, Lewis Base, Nucleophile, substrate, Electrophile, polar protic

solvent, polar aprotic solvent, inversion, rearrangements, leaving group, H-shift, Alkyl-

shift.

Acids and Bases:

• Identify acid, base, conjugate acid and conjugate base in a reaction.

• Which way does the equilibrium lie? Use pKa’s to determine, and if pKa is not available,

be able to use the stability of the conjugate base to determine the most acidic compound.

• Investigate the factors that affect the acidity of a compound: electronegative elements

near the element that donates a proton help stabilize a negative charge. If there is

resonance available to the compound once a proton has been removed, it will be more

acidic than a similar structure without resonance.

• For carbon-hydrogen bonds, the acidity of the H increases as the s character of the

Carbon increases. This means that an sp3 C-H bond is the least acidic (pKa about 50), sp2

C-H bonds are a little more acidic (pKa about 44), and the sp C-H bond would be the

most acidic type possible (pKa about 25).

• Tautomerization requires either acid or base to catalyze the ketone or aldehyde to the enol

form. For acid tautomerization, use H3O+, and for base tautomerization, use OH-. These

are always dissolved in water. Be able to draw the steps converting a Ketone to an enol.

• Tautomerization of Glucose to Fructose occurs in our cells by means of amino acids.

• Terms: Resonance, Arrhenius Acid, Arrhenius Base, Bronstead/Lowry Acid, Bronstead/

Lowry Base, Conjugate acid/base pair

Addition reactions:

• The alkene can be thought of as a source of electrons that can be donated. This would

make the alkene a nucleophile if it bonds to compound with a positive or partial positive

charge.

• There are many types of addition reactions, see your class notes for the major types you

will encounter on your exam.

• In general the addition of an electrophile to an alkene follows Markovnikov’s Rule: The

addition reaction must go through the most stable intermediate to proceed to the products.

This means the most stable carbocation must always be made, if the reaction creates one.

• Anti-Markovnikov product means that the group that added to the alkene (for example,

an OH group or halogen) is bonding to the less substituted carbon that was originally part

of an alkene bond. This does not necessarily mean that the less stable carbocation was

made, just that the product formed look as if that is what occurred. (see product of

sample question #7 below)

Nomenclature:

• The groups we have covered so far are the following: alkanes, alkenes, alkynes, alkyl

halides, ethers, benzene-containing compounds and small alcohols such as methanol and

ethanol. You should be able to use the IUPAC naming system to identify a structure

based on the name, and be able to recognize the correct name of a compound given a

structure.