Organic Chemistry: Intermolecular Forces, Solubility, and Reactions

Which of the following is not an intermolecular attractive force?

Covalent bond is not an intermolecular force; it is an intramolecular bond.

Which kind of compound usually has the highest boiling point among similar-size alkene, ketone, alcohol, and carboxylic acid?

Carboxylic acids usually have the highest boiling point because they can form especially strong intermolecular attractions.

Which kind of compound usually has the highest water solubility among similar-size hydrocarbon, ketone, alcohol, and alkene?

Alcohols are usually most water-soluble because they can hydrogen bond strongly with water.

Which hydrocarbon isomer has the highest boiling point?

The least branched or most straight-chain isomer usually has the highest boiling point.

Why does branching usually lower boiling point?

Branching lowers surface area and weakens London dispersion forces.

Why do alcohols usually boil higher than comparable alkyl halides or hydrocarbons?

Alcohols can hydrogen bond.

Why do larger halogens usually increase boiling point?

Larger halogens are more polarizable, so London forces are stronger.

What happens to boiling point as intermolecular forces get stronger?

Boiling point increases.

What usually dissolves better in water: lower alcohols or alkyl halides?

Lower alcohols dissolve much better in water than alkyl halides.

Why are alkyl halides usually poorly soluble in water?

They cannot hydrogen bond to water as effectively as alcohols and are relatively nonpolar.

What is a hydrogen bond donor?

Something with an H directly attached to N, O, or F that can donate that H in hydrogen bonding.

What is a hydrogen bond acceptor?

Something with a lone pair that can accept a hydrogen bond.

Can an alcohol hydrogen bond with water?

Yes; an alcohol can both donate and accept hydrogen bonds.

What makes a compound more polar overall?

More uneven charge distribution and the presence of polar bonds or polar functional groups.

What is the general polarity ranking logic?

More heteroatoms and stronger dipoles usually mean more polarity.

What is the general boiling point ranking logic?

Stronger intermolecular forces and less branching usually give higher boiling point.

What is the general water-solubility ranking logic?

More ability to hydrogen bond and less hydrocarbon character usually give higher solubility in water.

What is the stationary phase in TLC?

Usually silica gel, which is polar.

What is the mobile phase in TLC?

The solvent or solvent mixture that travels up the plate.

Why does solvent move up a TLC plate?

By capillary action.

What can TLC tell you about a sample?

TLC can help show purity and the number of components in a sample.

What is the definition of Rf?

Rf is how far the spot traveled relative to how far the solvent front traveled.

On silica TLC, which compound usually has the lowest Rf?

The most polar compound usually has the lowest Rf because it sticks most strongly to silica.

On silica TLC, which compound usually has the highest Rf?

The least polar compound usually has the highest Rf.

If the solvent becomes more polar on silica TLC, what happens to Rf values?

Rf values usually increase because the solvent competes better with silica.

Why does a more polar solvent usually make spots travel farther on silica TLC?

It pulls compounds off the polar silica more effectively.

Why is benzoic acid lower on silica TLC than bromobenzene or benzophenone?

Benzoic acid is more polar and hydrogen-bonds strongly to silica.

What does one spot on TLC usually suggest?

A relatively pure sample.

What do multiple spots on TLC usually suggest?

A mixture or impurity.

What is the purpose of extraction?

To separate compounds based on how they partition between two immiscible liquids.

Why do you vent a separatory funnel?

To release pressure built up during shaking.

What reagent moves benzoic acid from an organic layer into the aqueous layer?

Aqueous base such as NaOH converts it to a charged carboxylate salt.

What is the top vs bottom extraction layer rule?

The denser liquid is on the bottom.

Where is DCM in an extraction with water?

Usually the bottom layer because DCM is denser than water.

How can you tell which layer is aqueous?

Add a drop of water and see which layer it joins.

What is the purpose of recrystallization?

To purify a solid by dissolving it hot and crystallizing it on cooling.

What is the main function of a condenser in distillation?

To cool vapor back into liquid.

What does distillation separate by?

Differences in volatility and boiling point.

What is the Lucas test used for?

To classify alcohols by how readily they form alkyl halides under acidic conditions.

What is the Tollens test used for?

To identify aldehydes.

What test identifies unsaturation in an alkene?

Bromine in CCl4 decolorization indicates unsaturation.

What type of mechanism is common for dehydration of primary alcohols under strong acid and heat?

Often E2-like or concerted elimination because a primary carbocation is unstable.

The major alkene in dehydration usually follows what rule?

Zaitsev rule: the more substituted alkene is usually favored.

Why is the more substituted alkene usually favored in dehydration?

It is more thermodynamically stable.

Which reagent is not suitable for converting 2-methylcyclohexanol to 1-methylcyclohexene?

Aqueous NaOH is not the standard reagent for acid-catalyzed dehydration.

What kinds of reagents are suitable for acid-catalyzed dehydration?

Strong acids such as phosphoric acid or sulfuric acid.

What is the major product of dehydrating 2-methylcyclohexanol?

The more substituted alkene, 1-methylcyclohexene.

What is the logic for predicting the major product of dehydration reactions?

Expect the more substituted alkene unless steric or mechanistic factors strongly change the outcome.

What is the definition of a nucleophile?

An electron-pair donor.

What is the definition of an electrophile?

An electron-pair acceptor.

Which is a nucleophile: H2O, BF3, AlCl3, or CO2?

H2O is the nucleophile because it can donate a lone pair.

Are BF3 and AlCl3 electrophiles?

Yes, both are Lewis acids and act as electrophiles.

Which solvent is suitable for nucleophilic substitution among hexane, toluene, DMSO, and cyclohexane?

DMSO.

Why is DMSO a good solvent for many nucleophilic substitutions?

It is a polar aprotic solvent that boosts nucleophile reactivity.

What is the best solvent type for SN2?

Polar aprotic.

What is the best solvent type for SN1?

Polar protic, as it helps by stabilizing ions.

What does an SN2 reaction rate depend on?

Both substrate and nucleophile concentration.

What does an SN1 reaction rate depend on?

Only substrate concentration in the rate-determining step.

Describe the SN2 mechanism.

One-step backside attack with simultaneous bond making and bond breaking.

Describe the SN1 mechanism.

Two-step mechanism through a carbocation intermediate.

What is the stereochemistry of an SN2 reaction?

Backside attack causes inversion at the reacting center.

What is the stereochemistry of an SN1 reaction?

Often gives racemization or partial racemization because a planar carbocation is formed.

What is the best substrate for SN2?

Methyl then primary substrates are best.

Why are tertiary substrates bad for SN2?

Steric hindrance blocks backside attack.

What is the best substrate for SN1?

Tertiary substrates are best because they form the most stable carbocations.

What is the carbocation stability trend?

Tertiary > secondary > primary > methyl.

Why do more substituted carbocations react faster in SN1?

They are more stable.

Which substrate undergoes faster SN1: primary, secondary, or tertiary alkyl halide?

Tertiary alkyl halide.

Which leaving group is best among chloride, bromide, iodide, and tosylate?

Tosylate is excellent; among halides, iodide is usually best.

Which primary substrate reacts fastest with CN- among primary chloride, bromide, iodide, and tosylate?

Primary tosylate reacts fastest because tosylate is the best leaving group.

What is the leaving group ability trend among halides?

Iodide > bromide > chloride.

What is the general condition favored by the Diels-Alder reaction?

A thermal pathway.

What type of reaction is the Diels-Alder?

Concerted pericyclic cycloaddition.

What conformation must the diene adopt for Diels-Alder?

It must be able to adopt the s-cis conformation.

What is the best diene for Diels-Alder?

Electron-rich diene.

What is the best dienophile for Diels-Alder?

Electron-poor alkene or alkyne.

Why is the Diels-Alder reaction useful?

It forms a six-membered ring and new sigma bonds in one step.

What is the difference between endo and exo in Diels-Alder?

Endo places the dienophile substituent oriented under or toward the developing pi system; exo points away.

Which Diels-Alder product is usually favored kinetically?

The endo product.

Why is the endo product often favored?

Secondary orbital interactions stabilize the endo transition state.

What is the concept of reduction of trans-cinnamic acid?

Reduction removes the alkene unsaturation to give the more saturated carboxylic acid product.

What happens in catalytic hydrogenation or alkene reduction?

A pi bond is converted into a sigma bond by addition across the double bond.

What type of reaction is nitration?

Aromatic electrophilic substitution.

What is the electrophile in aromatic nitration?

The nitronium ion.

What intermediate is formed in the nitration of toluene?

The benzenium ion or arenium ion.

What directing effect does an ester group such as CO2Me have on an aromatic ring?

It is deactivating and meta-directing.

What is the major product of nitration of methyl benzoate?

The meta-nitro product.

What directing effect does a methyl group have in aromatic substitution?

It is activating and ortho/para-directing.

What directing effect does a carboxylic acid or ester carbonyl group have in aromatic substitution?

It is deactivating and meta-directing.

What is the definition of a Grignard reagent?

An organomagnesium halide that behaves like a strong carbon nucleophile and strong base.

What solvent is used to prepare a Grignard reagent?

Dry ether such as diethyl ether.

Why must Grignard reactions be dry?

Water, alcohols, and acids destroy the reagent by protonation.

What do Grignard reagents react with?

Carbonyl compounds.

What does a Grignard reagent do to an aldehyde or ketone?

It adds carbon to the carbonyl carbon and after workup gives an alcohol.

What does a Grignard reagent plus ketone usually give after workup?

A tertiary alcohol (if the ketone is not formaldehyde).

What is the product of salicylic acid plus acetic anhydride under acid?

Aspirin.

What is the nucleophile in aspirin synthesis?

The phenolic oxygen of salicylic acid.

What is the electrophile in aspirin synthesis?

Acetic anhydride.

What is the role of sulfuric acid in aspirin synthesis?

Acid catalyst that activates the acylating agent.

What functional group transformation occurs when aspirin is made?

Formation of an ester from the phenolic OH of salicylic acid.