PCAT Chemical Processes: Organic Chemistry Alkanes, Alkenes, and Alkynes

Oxidation of Potassium Permanganate

Alkenes can be oxidized with KMnO4 to provide different types of products depending on the reaction conditions. Cold, dilute, aqueous KMnO4 reacts to produce 1,2 diols (vicinal diols) which are also called glycols with syn orientation. Under acidic conditions, manganate ions (MnO4-) are reduced to manganese ions and this reaction can be coupled to the complete cleavage of a double bond In a hot, basic solution of KMnO4 is added to the alkene and then acidified, the product will be determined by nature of the substrate.

Alkane Physical Properties

As the molecular weight of a straight chain alkane increases, the melting point, boiling point, and density also increase. At room temp C1 alkanes to C4 alkanes are gases, C5 alkanes to C16 alkanes are liquids and C17+ are waxes and harder solids. Branched molecules have slightly lower boiling point and melting point than straight-chain because greater branching reduces surface area and intermolecular forces.

Mechanism of SN2 Reactions

Concerted reactions, entire mechanism occurs in a single coordinated process. The nucleophile attacks the reactant from the backside of the leaving group forming a trigonal bipyramidal transition state. As the reaction progresses, the bond to the nucleophile strengthens while the bond to the leaving group weakens. The leaving group is displaced as the bond to the nucleophile becomes complete.

Polymerization

Creation of long, high molecular weight chains (polymers). Usually occurs through a radical mechanism, anionic and cationic are commonly observed too.

SN2 Reactions

Designates a bimolecular nucleophilic reaction, reactions involving a nucleophile pushing its way into a compound while displacing the leaving group. Its rate is determined by the substrate and nucleophile concentrations

SN1 Reactions

Designates a unimolecular nucleophilic substation reaction. The rate of the reaction is only dependent upon one molecule, aka first order reaction. The rate-determining step of an SN1 reaction is the disassociation of the substrate (the starting molecule) to form a stable, positively charged ion called a carbocation. Its concentration determines rate of whole reaction. The formation and stabilization of the carbocation determines all other aspects of the reaction.

Hydroboration of Alkenes

Diborane, B2H6, adds readily to double bonds. The B atom is a Lewis acid and attached to the C. An oxidation-hydrolysis with a peroxide and aqueous base produces an alcohol with a syn orientation

Alkenes

Hydrocarbons that contain carbon-carbon double bonds. The general formula for a straight chain alkene with one double bond is CnH2n. The degree of unsaturation (number of N double bonds or rings) of a compound of molecular formula CnHm can be determined according to the equation N=1/2(2n+2-m) The carbons at either end of a double bond are sp^2 hybridized and both form planar bonds with bond angles of 120. Alkenes are not able to rotate around their double bond and thus are constrained to unique configurations.

Alkenes and Alkynes

Hydrocarbons that contain double and triple bonds between carbons. They are called unsaturated because they can contain fewer than the maximum possible number of hydrocarbons. Double and triple bonds are considered functional groups. Alkenes and alkynes are more reactive than alkanes. The double and triple bonds of alkenes and alkynes are made from first forming a single sigma bond and then by forming one or two additional pi bonds.

Reduction of Alkynes

Hydrogenated with a metal catalyst to produce alkanes in a reaction that goes to completion. A more useful reaction stops the reduction after addition of just one equivalent of H2, producing alkenes. This partial hydrogenation can take place using Lindlar's catalyst, a poison that stops the reaction at the alkene stage. The product is a cis isomer. The other method uses sodium below the liquid boiling point of ammonia and produce the trans isomer of the alkene via a free radical mechanism.

Basicity of Nucleophiles

If a group of nucleophiles are based on the same atoms they are roughly correlated to basicity. The stronger the base, the stronger the nucleophile. Since stronger bases are stronger e- donors, so are stronger nucleophiles. Example: RO- > HO- > RCO2- > ROH > H2O

Size and Polarizability of Nucleophiles

If a series of nucleophiles are based on different atoms nucleophilic ability does not necessarily correlate to basicity. In a protic solvent (one that is able to form hydrogen bonds) large atoms or ions are better nucleophiles since large ions are more polarizable. *Example: CN- > I- > HO- > Br- > Cl- > F- > H2O In an aprotic solvent (one that cannot form hydrogen bonds) the nucleophiles are naked, not solvated. Nucleophilic strength is related to basicity again. *Example: F- > Cl- > Br- > I-

Mechanism of SN1 Reactions

1. Disassociation of substrate into carbocation and leaving the group *Carbocations are stabilized by polar solvents with lone pairs to donate (water, ethyl alcohol). More highly substituted carbocations are more stable. *The original leaving group should be a weaker nucleophile than the replacement nucleophile 2. Combination of carbocation with nucleophile to form substituted product. Occurs very rapidly compared to first step, irreversible

Disproportionation

A radical transfers a H atom to another radical forming an alkane and alkene

Hydroboration of Alkynes

Addition of boron to triple bonds occurs by the same method as addition of B to double bonds. Addition is syn and the B atom adds first. The B atom can be replaced with a proton from acetic acid to produce a is alkene.

Peroxycarboxylic Acids

Alkenes can be oxidized with CH3CO3H and mCPBA. The products formed are oxiranes (epoxides) and are highly reactive. This makes them ideal for further nucleophilic reactions

Polarity of Alkenes

Polarity is a property that results from the asymmetrical distribution of e- in a particular molecule. In alkenes, this distribution creates dipole moments that are oriented from the electropositive alkyl groups toward the electronegative alkene. Trans have no net dipole movement, but cis do.

Free Radical Addition to Alkynes

Radicals add to the triple bonds as the do to double bonds. The reaction product is usually the trans isomer because the intermediate vinyl radical can isomerize to a more stable form

Combustion

Reaction allows alkanes with molecular oxygen to form CO2, H2O, and heat. It has a very complex mechanism and is believed to occur in a radical process. All combustion reactions take this same basic form with different hydrocarbons as start molecule. It is often incomplete, producing lots of CO rather than CO2 (occurs in burn of gas in engines)

Stereochemistry of SN2 Reactions

Single step of SN2 reaction involves a chiral transition state. Since the nucleophile attacks from one side of central carbon and leaving group departs the other, the reaction "flips" the bonds attached to the C. If reactant is chiral, optical activity will be retained but will invert between R and S if nucleophile and leaving group have the same priority as rest of molecule

Physical Properties of Alkynes

The shorter chain compounds are gases, boiling at somewhat higher temps than corresponding alkenes. The asymmetrical distribution of electron density causes alkynes to have dipole moments that are larger than those of alkenes, they are slightly polar. Terminal alkynes are relatively acidic

Ozonolysis

Treatment of alkenes with ozone followed by reduction of zinc and water is a milder reaction. This results in the cleavage of the double bond to produce 2 aldehyde molecules.

Synthesis of Alkynes

Triple bonds can be made by the elimination of 2 molecules of HX. This reaction is not always practical, requires high temps and strong base. A terminal triple bond is converted to a nucleophile by removing the acidic proton with strong base, producing an acetylide ion. This ion will perform nucleophilic displacements on alkyl halides at room temp using an SN2 mechanism

Unimolecular Elimination

Two-step process proceeding through a carbocation intermediate. The rate of the reaction depends only on the concentration of only the substrate. 1. the leaving group departs producing a carbocation 2. A proton is removed by a base and a double bond forms. E1 favors the same factors that favor SN1: protic solvents, highly branched carbon chains, good leaving groups, and weak nucleophiles in low concentration. High temps are favored too E1 and SN1 are competitive and occur simultaneously under the same conditions. They have the exact same rate law.

Characterizing Carbons Atoms

by the number of other carbon atoms they are directly bonded to. 1. A primary carbon (written as 1 followed by degree sign) is bonded to only one other carbon. 2. A secondary carbon (2 degree) is bonded to two C 3. Tertiary carbon (3 degree) is bonded to three C 4. Quaternary carbon (4 degree) is bonded to four C Hydrogens or functional groups attached to a carbon are referred to as the same as the C. (a hydrogen attached to a primary carbon is a primary hydrogen)

Alkanes

hydrocarbons with the maximum number of hydrogen attached to each carbon, meaning they are saturated. These compounds are only made up of carbon hydrogen single bonds with the general formula CnH2n+2. They may be modified with additional functional groups as well such as alcohols, halogens and amines.

Nucleophiles

molecules that are attracted to positive charge. They are electron-rich species that are often, but not always, negatively charged. They are attracted to atoms with full or partial positive charge.

Free Radical Substitution Reactions

Alkanes can react in this way where 1 or more H atoms are replaced by Cl, Br, or I atoms. Larger alkanes have many H a free radical can attack so they primarily attack the H on the C with the most substituents, most stable. 1. Initiation: Diatomic halogens are cleaved by UV light or peroxide (ROOH) resulting in formation of free radicals, uncharged species with unpaired electrons that are extremely reactive and readily attack alkanes 2. Propagation: Radical produces another radical that can continue the reaction. Free radical reacts with alkane to remove an H atom and create a alkyl radical. Alkyl radical can then react with X2 to form alkyl halide (the substituted product) and propagate the radical 3. Termination: Two free radicals combine to form a stable molecule. A single free radical can initiate many reactions before the reaction chain is terminated.

Substitution Reactions of Alkanes

Alkyl halides and other substituted carbon molecules can take part in reactions known as nucelophilic substitutions, removing an atom or functional group from a molecule and replacing it with another.

Oxidation of Alkynes

Alkynes can be oxididatively cleaved with either basic potassium permanganate (followed by acidification) or ozone. Carboxylic acids are produced

Free Radical Additions to Alkenes

An alternate mechanism exists for the addition of HX to alkenes. Proceeds through free-radical intermediates and occurs when peroxides, oxygen, or other impurities are present. Free radical additions add first to the double bond, producing the most stable free radical on the most substituted C. The H then adds to the free radical resulting in the less substituted product.

Nomenclature of Alkenes

Can be described by terms cis, trans, E and Z to configuration of functional groups around the double bond. The common names ethylene, propylene, and isobutylene are often used over the names ethane, propene, and 2-methyl-1-propene

Reduction Reactions of Alkenes

Catalytic hydrogenation is the reductive process of adding molecular hydrogen to a double bond with the aid of a metal catalyst (platinum, palladium, and nickel). The reaction takes place on the metal's surface. One face of the double bond is coordinated to the metal surface and the 2 H atoms are added to the same face. This is a syn addition and results in a meso compound if the starting molecule was symmetrical.

Electrophilic Additions to Alkenes

Compounds can add to the double bond while leaving carbon skeleton intact. The e- of the pi bond are particularly exposed and easily attacked by molecules that seek to accept an e- pair (Lewis acid). Because these groups are seeking that are electrophiles Addition of HX: The e- of the double bond act as a Lewis base and react with electrophilic HX. The first step yields a carbocation intermediate after the double bond reacts with a proton. In the second step, the halide ion combines with the carbocation to give an alkyl halide. The halogen will add to the carbocation and thus create the product with the halide on the most substituted C. Addition of X2: The addition of halogens to a double bond is a rapid process. The addition of bromine is used to test the presence of a double bond. Addition of H2O: Water can be added to alkenes under acidic conditions. The double bond is pronated to form the most stable carbocation. This carbocation reacts with water, forming a pronated alcohol which then loses a proton to yield the alcohol. The reaction is preformed at low temps

Electrophilic Addition to Alkynes

Electrophilic addition to alkynes in the same manner as it does to alkenes. The addition can be stopped at the intermediate stage or carried further

Alkynes

Hydrocarbon compounds that possess one or more carbon-carbon triple bonds. The general formula for a straight-chain alkyne with one triple bond is CnH2n-2. Triple bonds are sp hybridized and therefore linear.

Increasing Rate of SN1 Reactions

Increased by anything that promotes formation and stability of carbocation 1. By structural factors: highly substituted alkanes allow for distribution of positive charge over a greater number of H and C atoms, forming the most stable carbocation. Primary and methyl substrates do not typically react by SN1 mechanism 2. By solvent effects: highly polar solvents (water and alcohols) are better at surrounding and isolating ions than less polar solvents. 3. By nature of leaving the group: weak bases disassociate more easily from alkyl chain and make better leaving groups 4. By nature of nucleophile: does not require a strong nucleophile, run equally well with strong or weak

Bimolecular Elimination

Occurs in one step, its rate depends on the concentration of the base and substrate. A strong base removes a proton while simultaneously a halide ion anti to the proton leaves, resulting in formation of a double bond. Often there are 2 possible hydrogen that can be removed from carbons on either side of the leaving group, resulting in 2 products. Zaitsev's rule says the substituted double bond is formed preferentially. Controlling E2 vs. SN2 is easier than E1 vs. SN1 1. Steric hindrance does not greatly effect E2 reactions. Highly substituted, most stable alkenes, easily undergo E2 not SN2 2. A strong, bulky base favors E2 over SN2 but SN2 is favored over E2 by strong nucleophiles that are weak bases. Heat and basic conditions will result in an E2 mechanism. Heat and acidic conditions will result in E1

Pyrolysis

Occurs when a molecule is broken down by heat in the absence of oxygen. Also called cracking, most commonly used to reduce average molecular weight of heavy oils and increase production of desirable volatile compounds. In pyrolysis of alkanes, the C-C bonds are cleaved producing small-chain alkyl radicals that can recombine into a variety of alkanes

Nucleophiles Leaving Groups

The ease at which nucleophilic substitution takes pace is dependent on the nature of leaving the group. The best at leaving the group are weak bases because they can accept a negative charge and disassociate to form a stable ion.

Synthesis of Alkenes

The most common method involves elimination reactions of either alcohols or alkyl halides. In these reactions the molecule loses either HX (where X is a halide) or a molecule of water from 2 adjacent C to form a double bond. Elimination can occur by E1 (unimolecular) or E2 (bimolecular)

Increasing Rate of SN2 Reactions

The two species, nucleophile and substrate must "meet" in solution and increasing concentration of either makes a "meeting" more likely. SN2 follows second order kinetics so rate = k[substrate][nucleophile] 1. Structural Factors: Nucleophile must have unhindered access to central carbon of substrate. Substrates with little branching are best. So methyl > primary > secondary > tertiary (which typically don't react). This is opposite of SN1 2. Solvent Effects: SN2 reactions occur most readily in polar aprotic solvents, unable to form H bonds like DMSO. Because they cant form H bonds, they do not create solvation shell around nucleophile to interfere with its attack on substrate. 3. Nature of Leaving Group: Weak bases disassociate more easily from alkyl chain and make better leaving groups, increasing the ease of displacement by nucleophile 4. Nature of nucleophile: Must be a strong nucleophile to attack the substrate and displace the leaving group. Nucleophile is usually a negatively charged ion that a strong or weak base.

Physical Properties of Alkenes

Their melting and boiling points increase with increasing molecular weight and are similar in value to those of the corresponding alkanes. 1-alkenes (terminal alkenes) usually boil at a lower temp than internal alkenes. Trans-alkenes have a higher melting point than cis-alkenes because their higher symmetry allows better packing in the solid state. They also tend to have lower boiling points than cis-alkenes because they are less polar.