OCHEM Unit 2

For Iverson

What bonds are alkenes comprised of?

Alkenes have one sigma bond and one pi bond. The reactivity of alkenes are based on the pi bonds.

What are the most important consequences of a pi bond?

A double bond cannot rotate.

Electron density is above and below the bond axis.

All About Lewis Acids and Bases

A Lewis acid is any molecule or ion that can accept a pair of electrons to make a new covalent bond.

A Lewis base is any molecule or ion that can donate a pair of electrons (lone pair or pi bond) to make a new covalent bond.

(Note these definitions include proton transfer reactions in which the proton is the Lewis acid).

Some Lewis acids and Lewis bases will combine to form a strong covalent bond, but others form a complex with a weak type of covalent bond. The key interaction involves a lone pair from the Lewis base interacting with an electron deficient portion of the Lewis acid.

When a Lewis acid and Lewis base combine, the product is referred to as a Lewis acid-Lewis base complex. The bond is referred to as a "coordinate covalent bond" or "dative bond".

The stronger the acid the (higher/lower) the pKa

Lower

In an acid-base reaction, equilibrium favors formation of the (stronger / weaker) acid and the (stronger / weaker) base.

Weaker (side with higher acid pKa value).

In buffered aqueous solution, an acid will be present predominantly in its (protonated / deprotonated) state if the pH of the solution is a (smaller / larger) value than the pKa of the acid.

Deprotonated, Larger.

For acids that deprotonate to give an anion conjugate base, the (stronger / weaker) acid will produce the (more / less) stable anion conjugate base.

Stronger, More. In other words, when analyzing relative acid strength, compare the relative stabilities of the anion conjugate bases.

Five rules used when determining relative stabilities of anions (i.e. when you are predicting relative acid strength). ((These five rules are actually application of the following two principles: 1) negative charge is neutralized by nuclear positive charge and 2) delocalizing negative charge over a larger area is stabilizing.)):

a) Across a row of the Periodic Table, negative charge on a more electronegative atom is more stable. (Principle 1)

b) Down a column of the Periodic Table, larger anions are more stable than smaller ones. (This is more confusing than you think, so make sure you understand it). (Principle 2 dominates here)

c) Hybridization: The more S orbital character of the hybridization on the atom that has the negative charge, the more stable the anion (stability of anions is in the order sp>sp2>sp3) Principle 1)

d) Charges distributed over more atoms are better. (Nature hates isolated charges, so delocalizing a charge around by resonance is very stabilizing) (Pinciple 2)

e) Inductive Effect: Electronegative atoms such as F on atoms adjacent to the atom(s) with the negative charge will pull some of the charge away, thus spreading it out and leading to stabilization. (Principle 1 and 2)

What is the inductive effect?

Electronegative atoms such as F on atoms adjacent to the atom(s) with the negative charge will pull some of the charge away, thus spreading it out and leading to stabilization.

What is hyperconjugation?

The stabilizing interaction where electrons in a sigma (σ) bond (usually a C–H or C–C bond) delocalize into an adjacent unpopulated p-orbital, empty/partially filled orbital, or π-system.

What are the characteristics of a carbocation intermediate?

A carbocation intermediate has a carbon with a positive charge and three bonds. Neighboring alkyl groups donate electron density through inductive effects and hyperconjugation, spreading out the positive charge.

How does carbocation stability increase?

Tertiary (3°) → Secondary (2°) → Primary (1°) → Methyl (CH3)

What is Markovnikov’s Rule?

The halogen or nucleophile in the reaction attaches to the more substituted carbon and the H atom attaches to the carbon with more hydrogens.

What is an anti stereochemistry?

Two substituents add to opposite sides of the pi bond.

What is a syn stereochemistry?

Two substituents add to same sides of the pi bond.

What is a mixed stereochemistry?

Two substituents can add either to same or opposite sides of the pi bond.

Which stereochemistry is associated with enantiomers?

Any pair of molecules that have the opposite configuration. (R and S, S and R, RR and SS)

Which stereochemistry is associated with meso compound?

R,S = S,R for symmetric molecules.

What is a haloalkane?

A compound in which one or more hydrogen atoms have been replaced by halogen atoms (F, Cl, Br, I).

What is the intermediate in an alkene reaction with Hx to make haloalkanes?

Carbocation

What is the regiochemistry in an alkene reaction with Hx to make haloalkanes?

Markovnikov

What is the stereochemistry in an alkene reaction with Hx to make haloalkanes?

Mixed

What are the reactants in an acid catalyzed hydration mechanism?

H2O added in the presence of H2SO4 (a strong acid).

What is the intermediate in an acid catalyzed hydration reaction of an alkene to make alcohols?

Carbocation

What is the regiochemistry in an acid catalyzed hydration reaction of an alkene to make alcohols?

Markovnikov

What is the stereochemistry in an acid catalyzed hydration reaction of an alkene to make alcohols?

Mixed

What is halogenation?

A reaction that introduces one or more halogen atom (F, Cl, Br, I) into a compound.

What is a vicinal dihaloalkene?

A compound where two halogen atoms (F,Cl,Br,I) are bonded to neighboring carbon atoms in a chain.

What is the intermediate in a halogenation reaction of an alkene to give vicinal dihaloalkanes?

Three-members ring halonium ion.

What is the regiochemistry in a halogenation reaction of an alkene to give vicinal dihaloalkanes?

Not Applicable

What is the stereochemistry in a halogenation reaction of an alkene to give vicinal dihaloalkanes?

Anti

What is hydrohalogenation?

A reaction in which a halogen (F, Cl, Br, I)) are added with H2O to a carbon-carbon double or triple bond, breaking a pi bond and creating a carbon-hydroxide bond and a carbon-halogen bond.

What is a halohydrin?

A compound that has a halogen atom (F, Cl, Br, I) and a hydroxyl group (OH) on neighboring carbon atoms.

What is the intermediate in a hydrohalogenation reaction of an alkene to give halohydrins?

Three-membered ring halonium ion. Water will attack the more highly substituted carbon because that has more of a positive charge.

What is the regiochemistry in a hydrohalogenation reaction of an alkene to give halohydrins?

Markovnikov

What is the stereochemistry in a hydrohalogenation reaction of an alkene to give halohydrins?

Anti

What is the intermediate in a hydroboration-oxidation reaction of an alkene to give an alcohol?

Four-membered ring transition state. H and B atoms are added simultaneously to the same face of pi bond and the H atom goes on the more substituted carbon atom.

What is the regiochemistry in a hydroboration-oxidation reaction of an alkene to give an alcohol?

non-Markovnikov. Replace the pi bond with bonds to H on the more substituted carbon and OH on the less substituted carbon.

What is the stereochemistry in a hydroboration-oxidation reaction of an alkene to give an alcohol?

Syn.

What is a vicinal diol?

A compound that contains two hydroxyl groups (OH) attached to neighboring carbon atoms.

What is the intermediate in an “Ozzy Osbourne” reaction of an alkene to give a vicinal diol?

Cyclic osmate ester, makes it so both OH groups are added to the same face of the double bond

What is the regiochemistry in an “Ozzy Osbourne” reaction of an alkene to give a vicinal diol?

Not Applicable

What is the stereochemistry in an “Ozzy Osbourne” reaction of an alkene to give a vicinal diol?

Syn

What is ozonolysis?

A reaction in which ozone (O₃) spits carbon-carbon double or triple bonds, replacing them with either aldehydes, ketones, or both.

What is a aldehyde?

A compound with a double bonded oxygen with the O atom located at the end of a carbon chain and the terminal C atom is connected to an H.

What is a ketone?

A compound with a double bonded oxygen with the O atom located at any point in the carbon chain (typically in the middle) and the terminal C atom is NOT connected to an H.

What is the intermediate in a ozonolysis reaction of an alkene to give an aldehydes and ketones?

Five-membered ring. Malozonide that then rearranges into an ozonide intermediate, explaining why the carbon-carbon bond is broken.

What is the regiochemistry in a ozonolysis reaction of an alkene to give an aldehydes and ketones?

Not Applicable

What is the stereochemistry in a ozonolysis reaction of an alkene to give an aldehydes and ketones?

Not Applicable

What is the intermediate in a hydrogenation reaction of an alkene on a metal surface to give an alkane?

Surface-bound alkyl. Alkene and H2 adsorb on metal surface, then new bonds form to both carbon atoms essentially simultaneously so the H atoms add to the same face

What is the regiochemistry in a hydrogenation reaction of an alkene on a metal surface to give an alkane?

Not Applicable

What is the stereochemistry in a hydrogenation reaction of an alkene on a metal surface to give an alkane?

Syn