Hydrocarbons, Alcohols, and Substitutions Flashcards
Stability of Carbocation Intermediates
Alkyl groups, methyl, ethyl, and the like, are electron donating and carbocation-stabilizing because the electrons around the neighboring carbons are drawn towards the nearby positive charge, thus slightly reducing the electron poverty of the positively-charged carbon.
Carbons in Alkanes
Primary, Secondary, Tertiary, or Quaternary
Alkanes - Physical Properties
- Boiling point is governed by intermolecular forces. As straight chain carbons are added, molecular weight and intermolecular forces increase, thus increasing the boiling point and melting point of alkanes.
- Branching significantly lowers boiling point but increases melting point.
- Alkanes have the lowest density of all organic compounds. Density increases with molecular weight.
- Alkanes are soluble in benzene, carbontetrachloride, chloroform and other hydrocarbons.
- Alkanes are almost totally insoluble in water.
- Alkanes of four carbons or less are gases at room temperature.
Cycloalkanes
- Ring strain is zero for cyclohexane and increases as ring become larger (up to 9) or smaller. After 9, ring strain decreases to zero as more carbons are added to the ring.
< strain = lower energy and more stabilty
- Three conformers, all three exist at room temp.
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Chair
- Equatorial hydrogens (less crowding)
- Axial hydrogens
- Twist
- Boat
-
Chair
Combustion
Combustion takes place when oxygen is added to an alkane at high temperatures.
CH4 + 2O2 + flame → CO2 + 2H2O + Heat
Halogenation of Alkanes
Alkanes will react with halogens (F, Cl, Br, but not I) in the presence of heat or light to form a free radical. Energy from light or heat homolytically cleaves the diatomic halogen. The result is two highly reactive species each with an unpaired electron (free radical)
Halogenation:
- Initiation: Halogen is homolytically cleaved by heat or by UV light resulting in two free radicals.
- Propagation: The halogen radical removes a hydrogen from the alkane resulting in an alkyl radical. The alkyl radical may now react with a diatomic halogen molecle creating an 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.
Halogenation is an exothermic process.
Stability of Alkenes
Synthesis of Alkenes
Synthesis of an alkene occurs via an elimination reaction.
- Dehydration of an Alcohol (E1)
- Dehydrohalogenation (E1, weak base (B:)), (E2, strong base (B:-))
Carbocation rearrangement may occur to give the major product in E1 reaction
Dehydration of an Alcohol
Dehydration of an alcohol is an E1 reaction where an alcohol forms an alkene in the presense of hot concentrated acid.
- The acid protonates the hydroxyl group producing the good leaving group, water.
- Water drops off, forming a carbocation. Rearrangement may occur.
- A water molecule deprotonates the carbocation and an alkene is formed.
Dehydrohalogenation
Dehydrohalogenation may be E1 (absense of strong base) or E2 (a high concentration of a strong base, bulky base).
Catalytic Hydrogenation
With the presence of a metal catalyst, the H-H bond in H2 cleaves, and each hydrogen attaches to the metal catalyst surface, forming metal-hydrogen bonds. The metal catalyst also absorbs the alkene onto its surface. A hydrogen atom is then transferred to the alkene, forming a new C-H bond. A second hydrogen atom is transferred forming another C-H bond. At this point, two hydrogens have added to the carbons across the double bond. Because of the physical arrangement of the alkene and the hydrogens on a flat metal catalyst surface, the two hydrogens must add to the same face of the double bond, displaying syn addition.
Electrophilic Addition
When you see an alkene on the MCAT, check for electrophilic addition. When hydrogen halides (HF, HCl, HBr, and HI) are added to alkenes, they follow Markovnikov’s rule (hydrogens will add to the least substituted carbon of the double bond).
E1(two steps)
- The hydrogen halide, a Bronsted-Lowry acid, creates a positively charged proton, which acts as an electrophile and attacks the electron rich double/triple bond creating a carbocation.
- The carbocation picks up the negatively charged halide ion.
**** IF PEROXIDES (ROOR) ARE PRESENT, THE BROMINE, NOT THE HYDROGEN WILL ADD TO THE LEAST SUBSTITUTED CARBON (ANTI-MARKOVNIKOV). ALL OTHER HALOGENS WILL STILL FOLLOW MARKOVNIKOV’S RULE EVEN IN THE PRESENCE OF PEROXIDES.****
Halogenation of an Alkene
Halogens are much more reactive toward alkenes than toward alkanes, which require heat or light. Br2 and Cl2 add to alkenes readily via anti-addition to form vic-dihalides.
Hydration of an Alkene & Dehydration of an Alcohol
Hydration of an alkene takes place when water is added to an alkene in the presence of an acid. This reaction is the reverse of dehydration of an alcohol. Low temperatures and dilute acid drive this reaction toward alcohol formation; high temps and concentrated acid drive the reation toward alkene formation.
Benzene
Benzene undergoes substitution Not addition. If a benzene ring contains one substituent, the remaining 5 positions are labeled ortho, meta, or para.
- If attached substituent is an electron withdrawing group it directs all new substituents to the meta position.
- If attached substituent is an election donating group, it directs all new sustituents to ortho or para.
**** Halogens are an exception. They are ortho/para.***
SN1 reaction
SN1 reaction is a two-step reaction in which
- The leaving group leaves, forming a carbocation. This is the slow step, and so the rate is dependent only on the concentration of the substrate.
- The nucleophile attacks the carbocation. It can do this from either side, typically in a 50/50 ratio. Therefore about half the product has retained the original configuration, and about half is inverted.
- Protonated nucleophiles, such as methanol, then lose a proton to the solvent.
***On the MCAT, probably only tertiary substrates will undergo SN1***
Elimination (E1) often accompanies SN1 reactions because the nucleophile may act as a base to abstract a proton form the carbocation, forming a carbon-carbon double bond.
Solvents: Polar protic solvents such as water and alcohols stabilize the nucleophile, increasing the rate of an SN1.
Base Strength: Base strength is unimportant, since the base is not involved in the rate determining step (the formation of the carbocation). .
Leaving groups: A good leaving group is required, such as a halide or a tosylate, since the leaving group is involved in the rate-determining step.
Notes: Be wary of rearangements that can occur with the SN1 reaction. Because it goes through a carbocation intermediate, both hydrogen shifts and alkyl shifts can occur!
SN2 Reaction
The SN2 reaction is a single-step displacement of a leaving group by a nucleophile. During the transition state, the bond to the nucleophile forms at the same time that the bond to the leaving group breaks. Therefore the nucleophile is required to approach from the back, and configuration at carbon is inverted.
Solvents: Protic solvents such as water and alcohols stabilize the nucleophile so much that it won’t react. Therefore, a good polar aprotic solvent (can’t form hydrogen bonds) is required such as ethers and ketones and halogenated hydrocarbons.
Nucleophiles: A good nucleophile is required since it is involved in the rate-determining step.
Leaving groups: A good leaving group is required, such as a halide or a tosylate, since it is involved in the rate-determining step.
SN1 vs. SN2
“The nucleophile and the five Ss”
The nucleophile: SN2 requires a strong nucleophile, while nucleophile strength doesn’t affect SN1.
- SN2 reactions don’t occur with a sterically hindered Substrate. SN2 require a methly. primary or secondary substrate, while SN1 require a secondary or tertiary substrate.
- A highly polar Solvent increases the reaction rate of SN1 by stabilizing the carbocation, but slows down SN2 reactions by stabilizing the nucleophile.
- The Speed of an SN2 reaction depends upon the concentration of the substrate and the nucleophile, while the speed of an SN1 depends only on the substrate.
- SN2 invert Stereochemistry about the chiral center, while SN1 creates a racemic mixture.
- SN1 may be accompanied by carbon Skeleton rearrangement, but SN2 never rearranges the carbon skeleton.
Elimination can accompany both SN1 and SN2 reactions. Elimination occurs when the nucleophile behaves as a base, abstracting a proton rather than attacking a carbon. Elimination reactions always result in a carbon-carbon double bond.
Alcohols
Alcohols follow trends similar to hydrocarbons, but alcohols hydrogen bond, giving them considerably higher boiling points and water solubilities than similar-weight hydrocarbons.
- Boiling Point goes up with molecular weight and down with branching.
- Melting Point goes up with molecule weight.
Since alcohols can lose a proton, it can act like an acid. Placing an electron withdrawing group on the alcohol increases its acidity by reducing the negative chare on the conjugate base. For example, a tertiary alcohol has more methyl groups than a primary alcohol, a tertiary carbon can absorb less negative charge; the conjugate base of a tertiary alcohol is less stable; and a tertiary alcohol is less acidic than a primary alcohol.
Grignard Synthesis of an Alcohol
Organometallic reagents possess a highly polarized carbon-metal bond with the carbon more electronegative than the metal. This makes the carbon a strong nucleophile and base.
- The nucleophile attacks the carbonyl carbon.
- After an acid bath, an alcohol is produced.
- Grignard reagent will react in a sililar fashion with C_=_N, C=N, S=O, N=O.
- It is also a strong enough base to deprotonate the following: O-H, N-H, S-H, -C_=_C-H
- Grignard reagents are made in ether, and are incompatible with water and acids stronger than water.
Reduction Synthesis of an Alcohol
From NaBH4 or LiAlH4, hydrides (H-) wil react with carbonyls to form alcohols. Unlike Grignard synthesis of an alcohol, the use of hydrides does not extend the carbon skeleton.
**** Both NaBH4 and LiAlH4 will reduce aldehydes and ketones. Esters and acetates are strong electron donors that reduce the positive charge on the carbonyl carbon making it less attractive to the nucleophile. Only LiAlH4 is strong enough to reduce esters and acetates.
Reactions with Alcohols
Alcohols like to be nucleophiles
Oxidation of Alcohols
Oxidation: loss of H2; addition of O or O2; addition of X2
Reduction: addition of H2 (or H-); loss of O or O2; loss of X2
Neither: addition or loss of H+, H2O, HX, etc.
Primary alcohols>aldehydes>carboxylic acids
Secondary alcohols>ketones
Tosylates and Mesylates
Tosylates and Mesylates can be used to protect alcohols from unintended reactions.
Ethers
Ether is almost always the answer to solvent questions on the MCAT. They are polar and can hydrogen bond with compounds that contains hydrogen attached to a N, O, or F atom.
For the MCAT, ethers undergo one reaction. Ethers are cleaved by the halo-acids HI and HBr to form the corresponding alcohol and alkyl halide.
