Organic Chemistry Ch. 2

Front 1

Physical properties of alkanes

Back 1

1. As molecular weight increases, intermolecular forces increase and thus boiling point and melting point increases.
2. Branching of alkanes lowers the boiling point and increases the melting point.
3. Lowest density of all organic compounds. Density increases with molecular weight.
4. Almost totally insoluble in water.

Front 2

Ring strain

Back 2

-some ring structures put strain on the carbon-carbon bonds because they bend them away from the normal 109.5 angle of an sp3 carbon and cause crowding.
-Ring strain is ZERO for cyclohexane and increases as rings become larger or smaller.
-less ring strain = lower energy = more stability.

Front 3

Equatorial hydrogens

Back 3

-The hydrogens projecting outward from the center of the ring.

Front 4

Axial hydrogens

Back 4

-the hydrogens projecting upward or downward
-crowding occurs most often between groups in axial positions, thus most substituent groups are more favored in the equatorial position.

Front 5

Combustion

Back 5

-takes place when alkanes are mixed with oxygen and energy is added.
-only takes place at high temperatures.
-A radical reaction

SEE CARD FOR REACTION.

Front 6

Heat of Combustion

Back 6

-the change in enthalpy of a combustion reaction.
-Combustion of isomeric hydrocarbons requires equal amounts of oxygen and produces equal amounts of carbon dioxide and water
-Therfore, heats of combustion can be used to compare the relative stablities of isomers.
-The HIGHER the heat of combustion, the HIGHER the energy level of the molecule, the LESS stable the molecule.
-comparisons between molecules can be made by evaluating heat of combustion per CH2.

Front 7

Reactivity of alkenes

Back 7

-alkenes are more reactive than alkanes because they have pi-bonds which are less stable than sigma-bonds
-pi-bonds are electron hungery, explaining why alkenes are more acidic than alkanes
-attractive to electrophiles!

Front 8

Physical properties of alkenes

Back 8

-increase in MW = increase in BP
-Branching decreases BP.
-very slightly soluble in water
-lower density than water
-more acidic than alkanes.

Front 9

Elimination reaction

Back 9

-the synthesis of an alkene occurs via an elimination reaction
-one or two functional groups are eliminated or removed to form a double bond.
-this can occur via dehydration or dehydrohalongenation.

Front 10

E1 reaction

Back 10

-an elimination reaction; the rate of the reaction relies only on one species.

Front 11

Saytzeff rule

Back 11

-the major product of elimination will be the most substituted alkene.

Front 12

Syn-addition

Back 12

-same side addition reaction.
-occurs during catalytic hydrogenation of an alkene in the presence of Ni, Cd, Pt
-reduces an alkene to an alkane
-syn addition of an alkyne creates a cis alkene
-an exothermic reaction with a high energy of activation

Front 13

Electrophilic addition

Back 13

-The addition of an electron loving species to an alkene
-the electrons in the double bond of an alkene attract electrophiles
-the most reactive alkenes in electrophilic addition are the most thermodynamically stable because they have the lowest activation energy when forming carbocations.

Front 14

Electrophile

Back 14

-an electron loving species.

Front 15

Markonikov's rule.

Back 15

-When hydrogen halides are added to alkenes, the hydrogen will add to the least substituted carbon of the double bond.

Front 16

Hydration of an alkene

Back 16

-water and weak acid under cool conditions drives the alkene to react with water and form an alcohol.
-mechanism is the reverse of dehydration of an alcohol.

Front 17

Antiaddition

Back 17

-addition from opposite sides of the double bond
-occurs during oxymurcuration of an alkene.

Front 18

Substitution

Back 18

-a reaction that occurs when one functional group replaces another.

Front 19

SN1 reaction

Back 19

-2 step reaction, rate depends on only the substrate (formation of the carbocation).
-rate is independent of the nucleophile
-carbocation formation is the slow step.
-tertiary substrate is more likely to undergo SN1 because of stability of carbocation
-formation of both enantiomers will occur if there is a chiral carbon
-competes with E1.

Front 20

Sn2 reaction

Back 20

-reaction occurs in 1 step
-reaction rate is dependent on concentration of both the nucleophile and the substrate
-causes inversion of configuration
-If C is chiral, relative configuration may change, but absolute configuration may or may not
-tertiary carbon will sterically hinder this reaction. Rate of reaction is higher when less substituents are there
-If the nucleophile is a strong base and the substrate is too hindered, E2 may occur.
-Bulky nucleophiles hinder Sn2

Front 21

Strength of a nucleophile

Back 21

-strength of a nucleophile is unimportant for an SN1 reaction, important for an SN2 reaction.
-A base is always a stronger nucleophile than its conjugate acid.-
-a negative charge and polarizability adds to nucleophilicity
-generally increases and going right on the periodic table.
-electronegativity REDUCES nucleophilicity.

Front 22

polar protic solvents

Back 22

-polar solvents that can hydrogen bond
-Stabilizes the nucleophile and any carbocation that may form
-A stable nucleophile slows Sn2 reactions
-A stable carbocation increases the rate of SN1 reactions
-increases the rate of SN1 decreases the rate of SN2

Front 23

Polar aprotic solvents

Back 23

-do not hydrogen bond, don't form strong bonds with ions
-inhibit SN1, decrease rate of SN2

Front 24

Leaving groups

Back 24

-the best leaving groups are stable when they leave
-The weaker the base, the better the leaving group.
-Electron withdrawing = good leaving group
-leaving group will always be more stable than the nucleophile.

Front 25

SN1 vs. SN2

Back 25

1. The nucleophile- SN2 requires a strong nucleophile. nucleophilic strength does not effect SN1.
2. Substrate- SN2 doesnt occur with a sterically hindered substrate. requires a methyl, primary, or secondary, while SN1 requires a secondary or tertiary substrate.
3. Solvent- A highly polar solvent increases the reaction rate of SN1 by stabilizing the carbocation, slows down Sn2 by stabalizing the nucleophlie.
4. Speed- Speed of Sn2 depends on the concentration of both the substrate and nucleophile, SN1 depends on only the substrate.
5. Stereochemistry- SN2 inverts stereochemistry about the chiral center, SN1 creates a racemic mixture.
6. Skeleton-SN1 may be accompanied by carbon skeleton rearrangement but Sn2 never rearranges the carbon skeleton.

Front 26

substitution or elimination?

Back 26

-Elimination occurs when the nucleophile behaves as a base rather than a nucleophile; it abstracts a proton rather than attacking a carbon.
-E1 and E2 kinetics are similar to SN1 and SN2 kinetics respectively.

Front 27

Physical properties of alcohols

Back 27

-follow similar trends to alkenes
-bp goes up with mw, down with branching.
-bp is higher than alkanes because of hydrogen bonding, more imf's

Front 28

Alcohol as acid

Back 28

-Alcohol's can loose hydrogens, act like an acid.
-most stable conjugate base = strongest acid
-Excess charge leads to instability, most stable conjugate base will have the least negative charge.
-methyl groups are electron donating compared to hydrogens. More methyls lead to a more basic conjugate base, weaker acid.
-placing an electron withdrawing group increases alcohol acidity b/c reduces the negative charge on the conjugate base.

Front 29

How to tell if a species is oxidized

Back 29

-loss of H2, addition of O or O2, addition of X2.

Front 30

How to tell if a species is reduced

Back 30

-addition of H2 (or H-), loss of O or O2, loss of X2.

Front 31

Why ethers make good solvents

Back 31

-they are soluble in water, but organic compounds tend to be much more soluble in ethers than alcohols b/c no hydrogen bonds need to be broken.
-boiling points are roughly comparable to that of an alkane with a similar molecular weight. low boiling point is also useful for being a solvent.