2 - Hydrocarbons, Alcohols, and Substitutions

Question 1

Alkanes=

Answer 1

major functional groups contains only carbon-carbon single bonds.

—>methyl, primary, secondary, and tertiary depending on what is attached to them.

Question 2

—>Physical properties of Alkanes:

Answer 2

*as carbons are added to a single chain and molecular weight increases, thus boiling and melting point increases.

*Branching lowers boiling point though but increases melting point.

*Alkanes have the lowest density of all groups of organic compounds, density increases with molecular weight.

*Alkanes totally insoluble in water. But soluble in benzene, carbon tetrachloride, chloroform, and other hydrocarbons.

*If the alkane contains a polar functional group, the polarity and thus its solubility will decrease as the carbon chain is lengthened.

*the first four alkanes are gases at room temperature!

Question 3

Ring strain=

Answer 3

when the configurations start getting away from the 109.5 angles. *Ring strain is zero for cyclohexane and strain increases as rings become smaller or larger. The trend continues up to nine-carbon ring structures after which the strain decreases to zero as more carbons are added to the ring. Less ring strain means lower energy and more stability.

Question 4

*Cyclohexane exists as

Answer 4

chair, twist, and boat. All three exist at room temperature, but chair predominates completely because of it’s the lowest energy.

Question 5

Equatorial hydrogen=

Axial hydrogens= *

Answer 5

hydrogens projecting outward from the center of the ring.

hydrogens projecting upward or downward. *Crowding occurs most in this position; causing instability and raising energy level of the ring

Question 6

Combustion=

Answer 6

with sufficiently large energy of activation, they are capable of violent reactions with oxygen. *alkanes mixed with oxygen and energy (as well as high energy)= COMBUSTION* and once the combustion takes place it can make its own heat, and be self-perpetuating.

CH4 + 2O2 —(flame)—> CO2 + 2H20 (+ Heat)

Question 7

Radical Reaction=

Heat of combustion=

Free radical= .

Stability:

Answer 7

reactions like combustion and halogenation (both exothermic).

is the change in enthalpy of a combustion reaction.

alkanes will react with halogens (F, Cl, and Br, but not I) in the presence of heat or light to form these

tertiary>secondary>primary>methyl

Question 8

Halogenation (exothermic):
1)

2)

3)

Answer 8

Initiation: the halogen starts as a diatomic molecule, it is homolytically cleaved by heat or UV light, resulting in a free radical.

Propagation: halogen radical removes a hydrogen from the alkane resulting in an alkyl radical. Can now react with diatomic halogen molecule creating alkyl halide and a new halogen radical, propagation can continue indefinitely.

Termination: either two radicals bond or a radical bonds to the wall of the container to end the chain reaction or propagation.

Question 9

Alkene=

Answer 9

a carbon chain that contains a carbon-carbon double bond. *More reactive than alkanes because they have pie bonds.
***They follow the same rules for alkanes…***
*slightly soluble in water, and have a lower density than water.

Question 10

Elimination reaction=

Answer 10

the synthesis of an alkene. One or two functional groups are eliminated or removed to form a double bond. Base attacks a hydrogen…

Question 11

Dehydration of an alcohol=

Answer 11

is an E1 reaction where an alcohol forms an alkene in the presence of hot concentrated acid. *E1 means that the rate depends upon the concentration of only one of the species.
(concentration of the -OH)
1) acid protonates the -OH group producing the good leaving group, water. (fast step)
2) the water drops off, forming a carbocation (slow and rate determining step)
3) carbocation is formed, and rearrangement may occur.
4) water molecule deprotonates the carbocation and an alkene is formed.

Question 12

Carbocation stability=

Answer 12

tertiary, secondary, primary, and methyl.
–> rearrangement occurs only if a more stable carbocation can be formed.

Question 13

Saytzeff rule=

Answer 13

states that the major product of elimination will be the most substituted alkene.

Question 14

Dehydrohalogenation=

*E1=

*E2=

Answer 14

may proceed either E1 (absence of base) or E2 (strong bulky base)

the halogen drops off in the first step and a hydrogen is removed in the second step. (two steps)

the base removes a proton from the carbon next to the halogen-containing carbon and the halogen drops off, leaving an alkene. (one step)

Question 15

Syn-addition=

Anti-addition=

Answer 15

same side addition. As seen in catalytic hydrogenation which is an example of an addition reaction (exothermic with a high energy of activation) *The lower heat of hydrogenation the more stable the alkene. Syn-addition of alkynes also produce cis alkenes.

addition from opposite sides of the double bond.

Question 16

Ozonolysis=

Answer 16

oxidation, by ozone, to produce carboxyl groups with alkenes. With alkynes it produced carboxylic acids. Ozone contains reactive electron pairs with a high charge density, so it is very reactive, it breaks right through alkenes and alkynes.

Question 17

Electrophilic addition=.

Markovnikov’s rule=

Answer 17

important reaction for alkenes. Electrophile- an electron-loving species

when hydrogen halides are added to alkenes they follow this rule—>
“The hydrogen will add to the least substituted carbon of the double bond.”
1) the hydrogen halide, a bronsted-lowry acid, creates a positively charged proton, which acts as the electrophile. (Slow and rate determining)
2) The newly formed carbocation picks up the negatively charged halide ion.

Question 18

Anti-markovnikov addition=

Answer 18

if peroxides are present the halide not the hydrogen will add to the least substituted carbon (only the base of bromine being the halide).

Question 19

Hydration of an alkene=

*organometallic compounds, metals, like to lose

Answer 19

also follows markovnikov’s rule, hydration takes place when water is added to an alkene in the presence of an acid.
(low temp and dilute acid drive it towards alcohol formation, high temp and concentrated acid drive the reaction toward alkene formation.)

electrons and take on a full or partial positive charge.

Question 20

Halogenation of an alkene=

Benzene–>

Answer 20

notice that alkanes will not react with halogens without light or heat, but alkenes will. Alkynes behave just like alkenes when exposed to halogens.

undergoes substitution NOT addition. Don’t forgot the positions: ortho, meta, and para.

Question 21

Electron withdrawing group=

Electron donating groups=

Answer 21

is in the R position, it deactivates the ring and directs any new substituents to the meta position.

activate the ring and direct any new substituents to ortho and para positions.
*halogens are exceptions to this rule they deactivate the ring but are ortho-para directors.

Question 22

Substitution=

Sn1:

Sn2:

Answer 22

reactions occur when one functional group replaces another. Sn1 and Sn2

.-2 steps

• rate dependent on only one of the reactants
• the first step is the slow step and rate-determining step; which is the formation of the carbocation.
• it is directly proportional to the concentration of the substrate, not the nucleophile.
• the leaving group (group being replaced) simply breaks away on its own to leave a carbocation behind.
• 1 step
• rate is dependent on the concentration of the nucleophile and the substrate.
• nucleophile attacks the intact substrate from behind the leaving group and knocks the leaving group free while bonding to the substrate.
• there is an inversion of configuration.

Question 23

Sn1 VS Sn2 (nucleophile and 5 S’s)

Nucleophile:
1 S)=
2 S)=
3 S)=
4 S)=
5 S)=

***remember that elimination

Answer 23

Sn1 VS Sn2 (nucleophile and 5 S’s)

Nucleophile: Sn2 requires a strong nucleophile, while it doesn’t affect Sn1.
1 S)= Sn2 require a not sterically hindered substrate (methyl, primary, or secondary), while Sn1 requires secondary and tertiary.
2 S)= higher polar solvent increase the reaction rate of Sn1, but slows down Sn2 by stabilizing the carbocation in 1 and nucleophile in 2.
3 S)= Speed depends on the substrate and nucleophile in Sn2, and only the substrate in Sn1.
4 S)= Sn2 inverts stereochemistry about the chiral center, and Sn1 creates a racemic mixture.
5 S)= Sn1 may be accompanied by carbon Skeleton rearrangement, but Sn2 never rearranges the carbon skeleton.

_***_remember that elimination can also accompany Sn1 and Sn2 reactions. Elimination occurs when the nucleophile behaves as a base rather than a nucleophile, it abstracts a proton rather than attacking a carbon. Elimination reactions always result in a carbon-carbon double bond.

Question 24

Nucleophilicity=

Alcohols=

Answer 24

decreases going up and to the right on the periodic table.

follow the same general trends as alkanes.
-BP and MP goes up with molecular weight, and down with branching.
-MP not reliable as a trend.
*Much higher MP and BP, than alkanes, because of hydrogen bonding.
-More soluble in water, but the longer the carbon chain the less soluble in water.

*Alcohols like to be nucleophiles

*Primary and secondary alcohols can be oxidized, while tertiary cannot.

Question 25

Oxidation=
Reduction=
Neither oxi or red=

Answer 25

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

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

addition or loss of H+, H20, HX, etc…

Question 26

oxidation–>

<–reduction

Answer 26

Oxidation
—->
Alkane–> primary alcohol–>aldehyde–>carboxylic acid
Alkane–> secondary alcohol–>ketone
Alkane–> tertiary alcohol
<—-
Reduction

Oxidizing agents will have many more oxygens, while the reducing agents will have many hydrogens.

Question 27

Formation of the sulfonates=

Pinacol rearrangement=

Answer 27

nucleophilic substitution, where alcohol is the nucleophile. Alcohols form esters called sulfonates. Tosylates and Mesylates are commonly used sulfonates that you need to know for the MCAT

is a dehydaration of an alcohol that results in an expected product. Which finally ends up in a pinacolone formation.

Question 28

Ethers=

Answer 28

(other than epoxides) are relatively non-reactive. They are polar, cannot hydrogen bond with themselves but with other compounds that contain a H+ attached to N, O, or F atom. About as soluble in water as -OH. But organic compounds with no hydrogen bonds are much more soluble in water. *Ether is almost always the answer to solvent questions on the MCAT.
*an epoxide is an ether but is far more reactive!