Organic Chemistry Alkenes and Alkynes Flashcards
Double and triple ___ are considered functional groups, and alkenes and alkynes are more reactive than the corresponding alkanes
bonds
___ are hydrocarbons that contain carbon-carbon double bonds. The general formula for a straight-chain alkene with one double bond is C sub n H sub 2n. The degree of unsaturation (the number N of double bonds or rings) of a compound of molecular formula C sub n H sub m can be determined according to the equation:
N = (1/2)(2n+2-m)
The carbons at either end of a double bond are sp2 hybridized and both form planar bonds with bond angles of 120 degrees. Alkenes are not able to rotate around their double bond and thus are constrained to unique configurations
alkenes
Alkenes, also called ___, may be described by the terms cis, trans, E, and Z to distinguish the configuration of functional groups around the double bond. The common names ethylene, propylene, and isobutylene are often used over the IUPAC names ethene , propene, and 2-methyl-1-propene, respectively.
olefins
___ (or 1-alkenes) usually boil at a lower temperature than internal alkenes. Trans-alkenes generally have higher melting points 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
terminal alkenes
___ is a property that results from the asymmetrical distribution of electrons in a particular molecule. In alkenes, this distribution creates dipole moments that are oriented from the electropositive alkyl groups toward the electronegative alkene. In trans-2-butene, the two dipole moments are oriented in opposite directions and cancel each other. The compound possesses no net dipole moment and is not polar. On the other hand, cis-2-butene has a net dipole moment, resulting from addition of the two smaller dipoles. The compound is polar, and the additional intermolecular forces raise the boiling point
polarity
The most common method of ___ synthesis involves elimination reactions of either alcohols r alkyl halides. In these reactions the molecule loses either HX (where X is a halide) or a molecule of water from two adjacent carbons to form a double bond
alkene
Unimolecular ___, abbreviated E1, is a two step process proceeding through a carbocation intermediate. The rate of reaction is dependent on the concentration of only the substrate. The elimination of a leaving group and a proton results in the production of a double bond
elimination
Because both involve the formation of a ___ intermediate, E1 is favored by the same factors that factors that favor Sn1: protic solvents, highly branched carbon chains, good leaving groups, and weak nucleophiles in low concentration. After the identical first step, the reaction can proceed by either the E1 or SN1 pathway. E1 and SN1 are therefore competitive , and occur simultaneously under the same conditions. In fact, E1 and SN1 have exactly the same rate law, given by rate = k[substrate], including having the same value of k. Directing a reaction toward either E1 or SN1 alone is difficult, although high temperatures tend to favor E1; this means that a mixture of both E1 and SN1 products will always be present
carbocation
___, termed E2, occurs in one step. Its rate is dependent on the concentration of two species, the substrate and the base. A strong base such as the ethoxide ion (C2H5O-) removes a proton, while simultaneously a halide ion anti to the proton leaves, resulting in the formation of a double bond
biomolecular elimination
Often there are two possible ___ that can be removed from carbons on either side of the leaving group, resulting in two different product. In such cases, the more substituted double bond is formed preferentially this is known Zaitsev’s rule
hydrogens
Controlling E2 vs SN2 is easier than controlling E1 vs SN1:
1 ___ does not greatly affect E2 reactions. Therefore, highly substituted carbon chains, which form the most stable alkenes, undergo E2 easily and Sn2 rarely
2 A strong, bulky base such as t-butoxide favors E2 over SN2. SN2 is favored over E2 by strong nucleophiles that are weak bases such as CN- or I-
In general, heat and basic conditions will result in an E2 mechanism; heat and acidic conditions will result in E1 combined with SN1
steric hindrance
___ undergo several types of reactions including:
reduction by hydrogen, addition by electrophiles and free radicals, hydroboration, a variety of oxidations, and polymerization
alkenes
___ is the reductive process of adding molecular hydrogen (H2) to a double bond with the aid of a metal catalyst. Typical catalysts are platinum, palladium, and nickel (usually Raney nickel, a special powdered form), but on rare occasions rhodium, iridium, or ruthenium are used.
catalytic hydrogenation
The ___ takes place on the surface of the metal. One face of the double bond is coordinated to the metal surface, and thus the two hydrogen atoms are added to the same face of the double bond. This type of addition is called syn addition and results in a meso compound if the starting molecule was symmetrical
reaction
The ___ is somewhat weaker than the sigma bond, and can therefore be broken without breaking the sigma bond. As a result, compounds can add to double bonds while leaving the carbon skeleton intact. The electrons of the pi bond are particularly exposed and are thus easily attacked by molecules that seek to accept an electron pair (Lewis acids). Because these groups are electron-seeking they are known as electrophiles (literally, “lovers of electrons”)
pi bond
In the addition of HX, the electrons of the double bond act as a Lewis bas and react with electrophilic HX (hydrogen halide) molecules. The ___ 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. In cases where the alkene is asymmetrical, the initial protonation creates the most stable carbocation. The proton will add to the less substituted carbon atom (the carbon atom with the most hydrogens), and the positive charge of the carbocation will reside on the more substituted carbon. The halogen will then add to the carbocation, and thus create the product with the halide on the most substituted carbon. This phenomenon is called Markovnikov’s rule
first step
The ___ to a double bond is a rapid process. The addition of bromine and resulting change in color of the solution is frequently used as a diagnostic tool to test for the presence of double bonds. The double bond acts as a nucleophile and attacks a Br2 molecule, displacing Br-. The Br+ attaches to the double bond and a cyclic bronomium ion is formed. This is attacked by Br-, giving the dibromo compound. Note that this addition is anti because the Br- attacks the cyclic bronomium ion in a standard SN2 displacement from the opposite side. If the reaction is carried out in a nucleophilic solvent, the solvent molecules can compete in the displacement step, producing a bromo alcohol (for instance) rather than the dibromo compound.
addition of halogens
In the addition of H2O, water can be added to alkenes under ___. The double bond is protonated according to Markovnikov’s rule, forming the most stable carbocation. This carbocation reacts with water, forming a protonated alcohol, which then loses proton to yield the alcohol. The reaction is performed at low temperature because the reverse reaction is an acid-catalyzed dehydration favored by high temperatures. Direct addition of water is generally not useful in the laboratory because yields vary greatly with reaction conditions; therefore, this reaction is generally carried out indirectly using mercuric acetate
acidic conditions
An ___ exists for the addition of HX (hydrogen halide) to alkenes, which proceeds through free-radical intermediates, and occurs when peroxides, oxygen, or other impurities are present. Free radical additions are called anti-Markovnikov because X adds first to the double bond, producing the most stable free radical on the most substituted carbon. The hydrogen then adds to the free radial, resulting n the less substituted product. The reaction is useful for HBr but is not practical for HCl or HI because the energies are unfavorable
alternate mechanism
Diborane (B2H6) adds readily to ___. The boron atom is a Lewis acid and attaches to the less sterically hindered carbon atom. The second step is an oxidation-hydrolysis with peroxide and aqueous base, producing the alcohol with overall anti-Markovnikov, syn orientation
double bonds
___ can be oxidized with KMNO4 to provide different types of products, depending upon 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 2-) are reduced to manganese ions (Mn 2+), and this reaction can be coupled to the complete cleavage of a double bond. If a hot, basic solution of potassium permanganate is added to the alkene and then acidified, the product will be determined by the nature of the alkene substrate. Nonterminal alkenes are cleaved to form two molar equivalents of carboxylic acid, and terminal alkenes are cleaved to form a carboxylic acid and carbon dioxide. If the nonterminal double bonded carbon is disubstituted, however, a ketone will be formed
alkenes
___ with ozone followed by reduction with zinc and water is a milder reaction. This treatment results in cleavage of the double bond to produce two aldehyde molecules as shown (or ketones if the double bond is substituted with alkyl groups. If the reaction mixture is reduced with sodium borohydride, NABH4, the corresponding alcohols are produced.
treatment of alkenes
___ can be oxidized with peroxycarboxylic acids. Peroxyacetic acid (CH3CO3H) and m-chloroperoxybenzoic acid (mCPBA) are commonly used. The products formed are oxiranes (also called epoxides) and are highly reactive. This makes them ideal for further nucleophilic reactions such as substitutions and the formation of alcohols, glycols, and amines
alkenes
___ is the creation of long, high molecular weight chains (polymers), composed of repeating subunits (called monomers). Polymerization usually occurs through a radical mechanism, although anionic and even cationic polymerizations are commonly observed.
polymerization
___ are hydrocarbon compounds that possess one or more carbon-carbon triple bonds. The general formula for a straight-chain alkyne with one triple bond is C sub n H sub 2n-2. Triple bonds are sp hybridized and are therefore linear, imparting unique characteristics to molecules that contain them
alkynes
The suffix -yne is used to designate an alkyne, and the position of the triple bond is specified when necessary. A common exception to the IUPAC rules is ethyne, which is called ___. Frequently, compounds are named as derivatives of acetylene
acetylene
The ___ of the alkynes are similar to those of their analogous alkenes and alkanes. In general, the shorter-chain compounds are gases, boiling at somewhat higher temperatures than corresponding alkenes. Internal alkynes, like alkenes, boil at higher temperatures than terminal alkynes. Asymmetrical distribution of electron density causes alkynes to have dipole moments that are larger than those of alkenes, but still small in magnitude. Thus solutions of alkynes can be slightly polar. Terminal alkynes are relatively acidic, having pK sub a values of approximately 25
physical properties
___ can be made by the elimination of two molecules of HX from a geminal or vicinal dihalide. This reaction is not always practical and requires high temperatures and a strong base. A more useful method adds an already existing triple bond onto a particular carbon skeleton. 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 temperature using an Sn2 mechanism
triple bonds
___ undergo several types of reactions, most of which are closely related to the reactions of alkenes: a variety of reduction mechanisms by hydrogen and catalysts, addition by electrophiles and free radicals, hydroboration, and oxidation
alkenes
Alkynes, just like ___, can be 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 in two different ways. The first uses Lindlar’s catalyst, which is palladium on barium sulfate (BaSO4) with quinoline, a poison that stops the reaction at the alkene stage. Because the reaction occurs on a metal surface, the product alkene is the cis isomer. The other method uses sodium in liquid ammonia below -33 degrees Celsius (the boiling point of ammonia), and produces the trans isomer of the alkene via a free radical mechanism
alkenes
___ to alkynes occurs in the same manner as it does to alkenes, with the reaction following Markovnikov’s rule. The addition can generally be stopped at the intermediate alkene stage, or carried further
electrophilic addition
___ add to triple bonds as they do to double bonds - with anti-Markovnikov placement of the halogen. The reaction product is usually the trans isomer, because the intermediate vinyl radical can isomerize to its more stable form
radicals
___ of boron to triple bonds occurs by the same method as addition of boron to double bonds. Addition is syn, and the boron atom adds first. The boron atom can be replaced with a proton from acetic acid, to produce a cis alkene
Addition
With ___, a disubstituted borane is used to prevent further boration of the vinylic intermediate to an alkane. The vinylic borane intermediate can be oxidatively cleaved with hydrogen peroxide (H2O2), creating an intermediate vinyl alcohol, which rearranges to the more stable carbonyl compound via keto-enol tautomerism
terminal alkynes
___ can be oxidatively cleaved with either basic potassium permanganate (followed by acidification) or ozone. In both instances shown, carboxylic acids are produced, but treatment of a terminal alkyne with basic permanganate will produce carbon dioxide
alkynes
