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Addition of \(HBr\) to alkenes
The simplest type of electrophilic reaction to visualize is the addition of a haloacid such as \(HBr\) to an isolated alkene. It is not a biological reaction, but nonetheless can serve as a convenient model to introduce some of the most important ideas about electrophilic reactions.
Electrophilic addition of \(HBr\) to an alkene:
Step 1 is an acid-base reaction: the \(\pi \) electrons of the alkene act as a base and extract the acidic proton of \(HBr\). This leaves one of the carbons with a new bond to hydrogen, and the other with an incomplete octet and a positive formal charge. In step 2, the nucleophilic bromide anion attacks the electrophilic carbocation to form a new carbon-bromine bond. Overall, the \(HBr\) molecule - in the form of a proton and a bromide anion - has been added to the double bond.
To understand how \(\pi \)-bonded electrons in an alkene could be basic, let's first review the bonding picture for alkenes. Recall (section 2.1) that the both of the carbons in an alkene group are \(sp^2\) hybridized, meaning that each carbon has three \(sp^2\) hybrid orbitals extending out in the same plane at \(180^{\circ}\) angles (trigonal planar geometry), and a single, unhybridized \(p\) orbital oriented perpendicular to that plane - one lobe above the plane, one lobe below.
The unhybridized \(p\) orbitals on the two alkene carbons overlap, in a side-by-side fashion, to form the \(pi \) bond, which extends above and below the plane formed by the \(s\) bonds. two electrons shared in this π bond are farther away from the carbon nuclei than the electrons in the carbon-carbon \(s\) bond, and thus are more accessible to the acidic proton. In addition, recall that molecular orbital (MO) theory tells us that \(p\) orbitals are higher in energy than s orbitals (section 2.2). As a consequence, it is easier to break the \(p\) bond of an alkene than it is to break the \(s\) bond: the \(p\) bond is more reactive.
As the \(HBr\) molecule approaches the alkene, a new \(s\) bond is formed between one of the alkene carbons and the electron-poor proton from \(HBr\). The carbon, which was \(sp^2\) hybridized when it was part of the alkene, is now \(sp^3\) hybridized. The other alkene carbon is still \(sp^2\) hybridized, but it now bears a positive formal charge because it has only three bonds, and its \(p\) orbital is empty. But it won't stay empty for long: a carbocation is a very reactive, unstable intermediate. The bromide ion will rapidly act as a nucleophile, filling the orbital with a pair of electrons, and now with four \(s\) bonds the carbon is \(sp^3\)-hybridized.
The first step in the electrophilic addition reaction is much slower than the second step, because the intermediate carbocation species is higher in energy than either the reactants or the products, and as a result the energy barrier for the first step is also higher than for the second step. The slower first step is the rate-determining step: a change in the rate of the slow step will effect the rate of the overall reaction, while a change in the rate of the fast step will not.
It is important to recognize the inherent difference between an electrophilic addition to an alkene and a conjugate addition to an alkene in the \(\square \square \square\) position, the latter of which we studied earlier in section 13.4. In both reactions, a proton and a nucleophile add to the double bond of an alkene. In a conjugate addition, the nucleophilic attack takes place first, resulting in a negatively charged intermediate (an enolate). Protonation is the second step. Also, of course, the alkene must be conjugated to a carbonyl or imine.
In an electrophilic addition, proton abstraction occurs first, generating a positively-charged intermediate. Nucleophilic attack is the second step. No conjugated carbonyl or imine group is required: in fact a nearby carbonyl group would actually slow down a hypothetical electrophilic addition reaction down because a carbonyl is an electron withdrawing, carbocation-destabilized group.
The stereochemistry of electrophilic addition
Depending on the structure of the starting alkene, electrophilic addition has the potential to create two new chiral centers. Addition of \(HBr\) to an alkene is not stereoselective: the reaction results in racemization at both of the alkene carbons. Consider the addition of \(HBr\) to cis-3,4-dimethyl-3-hexene. The initial proton abstraction step creates a new chiral center, and because the acidic proton could be added to either side of the planar alkene carbon with equal probability, the center could have either \(S\) or \(R\) configuration.
Likewise, in the second step the nucleophilic bromide ion could attack from either side of the planar carbocation, leading to an equal mixture of \(S\) and \(R\) configuration at that carbon as well. Therefore, we expect the product mixture to consist of equal amounts of four different stereoisomers.
Exercise 14.2.1
Predict the product(s) of electrophilic addition of \(HBr\) to the following alkenes. Draw all possible stereoisomers that could form, and take care not to draw identical structures twice.
- trans-2-butene
- cis-3-hexene
- cyclopentene
The regiochemistry of electrophilic addition
In many cases of electrophilic addition to an alkene, regiochemistry comes into play: the reaction can result in the formation of two different constitutional isomers. Consider the electrophilic addition of \(HBr\) to 2-methylpropene:
Note that carbon #1 and carbon #2 in the starting alkene are not the same - carbon #2 is bonded to two methyl groups, and carbon #1 to two hydrogen atoms. The initial protonation step could therefore go two different ways, resulting in two different carbocation intermediates. Notice how pathway 'a' gives a tertiary carbocation intermediate (\(I_a\)), while pathway 'b' gives a primary carbocation intermediate (\(I_b\)) We know from section 8.5 that the tertiary carbocation \(I_a\) is lower in energy. Consequently, the transition state \(TS(a)\) leading to \(I_a\) is lower in energy than \(TS(b)\), meaning that \(I_a\) forms faster than \(I_b\).
Because the protonation step is the rate determining step for the reaction, tertiary alkyl bromide A will form faster than the primary alkyl bromide B, and thus A will be the predominant product of the reaction. The electrophilic addition of \(HBr\) to 2-methylpropene is regioselective: more than one constitutional isomer can potentially form, but one isomer is favored over the other. It is generally observed that in electrophilic addition of haloacids to alkenes, the more substituted carbon is the one that ends up bonded to the heteroatom of the acid, while the less substituted carbon is protonated. This 'rule of thumb' is known as Markovnikov's rule, after the Russian chemist Vladimir Markovnikov who proposed it in 1869.
While it is useful in many cases, Markovikov's rule does not apply to all electrophilic addition reactions. It is better to use a more general principle:
The regioselectivity of electrophilic addition
When an asymmetrical alkene undergoes electrophilic addition, the product that predominates is the one that results from the more stable of the two possible carbocation intermediates.
How is this different from Markovnikov's original rule? Consider the following hypothetical reaction, in which the starting alkene incorporates two trifluoromethyl substituents:
Now when \(HBr\) is added, it is the less substituted carbocation that forms faster in the rate-determining protonation step, because in this intermediate the carbon bearing the positive charge is located further away from the electron-withdrawing, cation-destabilizing fluorines. As a result, the predominant product is the secondary rather than the tertiary bromoalkane. This is referred to as an anti-Markovnikov addition product, because it 'breaks' Markovnikov's rule.
If the two possible carbocation intermediates in an electrophilic addition reaction are of similar stability, the product will be a mixture of constitutional isomers.
Electrophilic addition of water and alcohol
The (non-biochemical) addition of water to an alkene is very similar mechanistically to the addition of a haloacid such as \(HBr\) or \(HCl\), and the same stereochemical and regiochemical principles apply. A catalytic amount of a strong acid such as phosphoric or sulfuric acid is required, so that the acidic species in solution is actually \(H_3O^+\). Note that \(H_3O^+\) is regenerated in the course of the reaction.
Figure 14.2.10
If an alkene is treated with methanol and a catalytic amount of strong acid, the result is an ether:
Figure 14.2.11
Exercise 14.2.2
Draw a mechanism for the ether-forming reaction above.
Addition to conjugated alkenes
Electrophilic addition to conjugated alkenes presents additional regiochemical possibilities, due to resonance delocalization of the allylic carbocation intermediate. Addition of one molar equivalent of \(HBr\) to 1,3-butadiene, for example, leads to a mixture of three products, two of which are a pair of enantiomers due to the creation of a chiral center at carbon #2.
Figure 14.2.12
Exercise 14.2.3
Explain why 4-bromo-1-butene is not a significant product of the reaction above.
Exercise 14.2.4
Predict the major product(s) of the following reactions. Draw all possible stereoisomers, and take care not to draw the same structure twice.
- Hint - are the double bonds in an aromatic ring likely to undergo electrophilic addition?
Biochemical electrophilic addition reactions
Myrcene is an isoprenoid compound synthesized by many different kinds of plants and used in the preparation of perfumes. Recently an enzymatic pathway for the degradation of myrcene has been identified in bacteria (J. Biol. Chem 2010, 285, 30436). The first step of this pathway is electrophilic addition of water to a conjugated alkene system.
Figure 14.2.13
Exercise 14.2.5
Draw a mechanism for the above reaction, showing two resonance contributors of the carbocation intermediate. How would you characterize the intermediate?
Although the hydration of myrcene above looks very familiar, many enzyme-catalyzed electrophilic addition reactions differ from what we have seen so far, in that the electron-poor species attacked by the p-bonded electrons in the initial step is a carbocation rather than an acidic proton:
Figure 14.2.14
\(\square\)-terpineol, a major component in the sap of pine trees, is formed in an electrophilic addition reaction. The first thing that happens (which we will refer to below as 'step a', in order to keep the step numbering consistent what the addition mechanisms we have seen so far) is departure of a pyrophosphate leaving group, forming an allylic carbocation electrophile.
Figure 14.2.15
The actual electrophilic addition stage of the reaction begins with step 1, as the π electrons an alkene are drawn toward one of the two carbons that share the positive charge, effectively closing a six-membered ring. A water molecule then attacks the second carbocation intermediate (step 2), which completes the addition process.
Notice something important about the regiochemical course of the reaction: step 1 results in the formation of a six-membered ring and a tertiary carbocation. As we have stressed before, biochemical reactions tend to follow energetically favorable mechanistic pathways.
Exercise 14.2.6
An alternate regiochemical course to step 1 shown above could result in a seven-membered ring and a secondary carbocation, a much less energetically favorable intermediate in terms of both carbocation stability and ring size. Draw a mechanism for this hypothetical alternate reaction, and show the product that would result after the addition of water in a hypothetical 'step 2'.
FAQs
14.2: Electrophilic Addition to Alkenes? ›
Electrophilic addition of HBr to an alkene: Step 1 is an acid-base reaction: the π electrons of the alkene act as a base and extract the acidic proton of HBr. This leaves one of the carbons with a new bond to hydrogen, and the other with an incomplete octet and a positive formal charge.
What is the electrophilic addition of alkenes? ›An electrophilic addition reaction is a reaction in which a substrate is initially attacked by an electrophile, and the overall result is the addition of one or more relatively simple molecules across a multiple bond.
What is 14 addition? ›1,4-Addition is an electrophilic addition reaction of conjugate dienes. eg: Two electrophilic addition reactions could occur between 1,3-butadiene (1) and hydrogen chloride. In Reaction 1, the net reaction is addition of a hydrogen atom to C-1 and a chlorine atom to C-4 in 1.
What is the electrophilic addition of h2so4 to alkene? ›The electrophilic addition reaction between ethene and sulfuric acid. Alkenes react with concentrated sulfuric acid in the cold to produce alkyl hydrogensulphates. For example, ethene reacts to give ethyl hydrogensulphate.
What is the electrophilic addition of HBr to alkenes? ›Electrophilic addition reactions involving hydrogen bromide
Alkenes react with hydrogen bromide in the cold. The double bond breaks and a hydrogen atom ends up attached to one of the carbons and a bromine atom to the other. In the case of ethene, bromoethane is formed. With cyclohexene you get bromocyclohexane.
The electrophilic addition of HX to an alkene is said to follow Markovnikov's rule. Markovnikov's rule: During the electrophilic addition of HX to an alkene, the H adds to the carbon of the double bond with the fewest number of alkyl substitutent.
How do you identify an electrophilic addition reaction? ›An electrophilic addition reaction can be described as an addition reaction in which a reactant with multiple bonds as in a double or triple bond undergoes its π bond broken and two new σ bonds are formed.
Why is it called 1 4 addition? ›The reaction can also be called a 1,4-addition because the nucleophile added to position 4 of the α, β-unsaturated system.
Why does 1 4 addition occur? ›The “1,4 Addition” Product Of Acids Adding To Butadiene
All four carbons participate in the reaction. A new C-H single bond has formed on one end of the diene (C-1), and C-Br formed on the other end (C-4). Note that the C1-C2 and C3-C4 pi bonds are broken, and we've formed a new pi bond between C2 and C3.
What is 1 4 Addition? 1 4 addition or 1,4-addition is a type of organic chemical reaction that involves the addition of functional groups to the 1st and 2nd carbon atoms of a carbon chain. This means the functional groups in the product of the 1 4 addition reaction are attached to non-adjacent carbon atoms.
What is electrophilic addition of x2 to alkynes? ›
Electrophilic Addition of X2 to Alkynes
Alkynes undergo the same type of electrophilic addition with chloride and bromine as alkenes. However, with alkynes the halogen addition can occur once or twice depending on the molar equivalents of halogen used in the reaction.
Electrophilic addition reactions are an important class of reactions in chemistry. They hold such great value in the field of chemistry because they allow you to convert unsaturated hydrocarbons (alkene and alkynes) into another group of organic compounds such as alkanes, alcohols, alkyl halides, etc.
What is the product of electrophilic addition? ›The major product, 3-bromo-1-butene, results from electrophilic addition across the C-1 to C-2 double bond. This is a 1,2-addition reaction. The minor product, 1-bromo-2-butene, results from addition of HBr to the C-1 and C-4. This is a 1,4-addition reaction.
What is the electrophilic addition of cyclohexene? ›Cyclohexene reacts with bromine in the same way and under the same conditions as any other alkene. 1,2-dibromocyclohexane is formed. The reaction is an example of electrophilic addition.
What does HBr do to an alkene? ›HBr adds to alkenes to create alkyl halides. A good way to think of the reaction is that the pi bond of the alkene acts as a weak nucleophile and reacts with the electrophilic proton of HBr. Alternatively, you can view the first step of the reaction as the protonation of the pi bond.
What is electrophilic addition HBr to propene? ›Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yields 1-bromopropane.
What is electrophilic addition summary? ›Electrophilic addition is a reaction between an electrophile and nucleophile, adding to double or triple bonds. An electrophile is defined by a molecule with a tendency to react with other molecules containing a donatable pair of electrons.
What is the order of rate of electrophilic addition? ›Electrophilic addition reaction takes place in two steps. In step 1 the formation of carbocation takes place and in the second step this carbocation attacks electron rich atoms present near the double bond.
How many steps are electrophilic addition reaction? ›The first step involves the addition of an electrophile. Name the type of intermediate formed in the first step of the following addition reaction.
What is an example of an alkene addition reaction? ›What is an example of alkene addition reactions? Alkene addition reactions include the bromine test and production of ethanol. These reactions occur through hydration, halogenation, and halohydrin reactions.
How do you know if a reaction is electrophilic or nucleophilic addition? ›
The key difference between nucleophilic and electrophilic addition is that in nucleophilic addition reactions, an electron-rich component is added to a molecule, whereas in electrophilic addition, an electron-deficient species is added to a molecule.
What is the first step of electrophilic addition? ›The first step is electrophilic attack on the carbonyl pi bond by the electrophilic, acid H+. This step makes a carbocation, which is then attacked by the weak but very abundant nucleophile water.
What does 1,2 and 1 4 addition mean? ›Here, 1,2 indicates two neighbouring carbon atoms, while 1,4 indicates two carbon atoms which are separated in the carbon chain by two additional carbon atoms. Thus in 1,2- and 1,4-additions to 2,4-hexadiene, the additions actually occur at carbons 2 and 3, and 2 and 5, respectively.
What is the difference between 1 4 and 1,2-addition? ›The 1,2-addition has a smaller activation energy than 1,4-addition - it occurs faster than 1,4 addition, because the bromide nucleophile is closer to carbon #2 then to carbon #4. However, the 1,4-product is more stable than the 1,2-product.
Is a 1 4 addition reaction reversible? ›If the nucleophile is a strong base, such as Grignard reagents, both the 1,2 and 1,4 reactions are irreversible and therefor are under kinetic control.
What is the 1,2 addition carbonyl? ›The carbonyl carbon is an excellent electrophile and will undergo reactions with a large variety of nucleophiles. This reaction is called, “nucleophilic addition”, or sometimes, “1,2-addition”. In this reaction a C-Nu bond is formed and the C-O pi bond breaks.
How do addition reactions occur? ›Addition reaction occurs when two or more reactants combine to form a product without the loss of any atoms present in the reactants. Addition reaction is common in compounds that have unsaturated C-C bond, like double (alkene) and triple (alkyne) bonds. The weaker π bond is converted into two new stronger σ bonds.
Is the 1,2 adduct always the kinetic product? ›Therefore, the 1,2 adduct is the kinetic product as it forms faster but is less stable (check the energy diagram above).
What are the numbers in a addition? ›Addition in math is a process of combining two or more numbers. Addends are the numbers being added, and the result or the final answer we get after the process is called the sum.
What is a Dialkene? ›dialkene (plural dialkenes) (organic chemistry) Any alkene having two double bonds.
Why does 14 benzene undergo electrophilic substitution reactions though it has three double bonds justify? ›
Benzene is a planar molecule having delocalized electrons above and below the plane of the ring. Hence, it is electron-rich. As a result, it is highly attractive to electron-deficient species i.e., electrophiles. Therefore, it undergoes electrophilic substitution reactions very easily.
What are the 2 electrophilic substitution reactions? ›The two primary types of electrophilic substitution reactions undergone by organic compounds are electrophilic aromatic substitution reactions and electrophilic aliphatic substitution reactions.
What is electrophilic addition h2o? ›Electrophilic hydration is the act of adding electrophilic hydrogen from a non-nucleophilic strong acid (a reusable catalyst, examples of which include sulfuric and phosphoric acid) and applying appropriate temperatures to break the alkene's double bond.
Which alkene is most reactive towards electrophilic addition reaction? ›A : Alkynes is more reactive than alkene towards electrophilic addition reaction.
What is the difference between addition and electrophilic? ›A nucleophilic addition reaction has a nucleophile being added up. This nucleophile provides or donates electrons on the place of its addition. While an electrophilic addition reaction has an electrophile, which is an electron deficient species that accepts electrons.
What is the difference between electrophilic addition and substitution? ›Hence, nucleophilic substitution involves displacement by the nucleophile to form the product, while electrophilic addition involves the addition of the electron deficient electrophile on the electron dense site of the molecule.
Why is it called electrophilic addition? ›An electrophilic addition reaction is an addition reaction which happens because what we think of as the "important" molecule is attacked by an electrophile. The "important" molecule has a region of high electron density which is attacked by something carrying some degree of positive charge.
What is another name for electrophilic addition? ›Nucleophilic Addition
It is the process of adding an electrophile to the pi bond of an alkene. It is the process of adding a nucleophile to either an electron-deficient species or a pi bond in a molecule.
Electrophiles are electron-deficient species that are attracted to an electron-rich center. Electrophiles react by accepting an electron pair in order to form a bond to a nucleophile including the interactions of a proton and a base.
What is the test and positive result for an alkene? ›This allows us to distinguish alkenes from alkanes using a simple chemical test. Bromine water is an orange solution of bromine. It becomes colourless when it is shaken with an alkene. Alkenes can decolourise bromine water, but alkanes cannot.
What does Br2 do to an alkene? ›
Description: Treatment of alkenes with bromine (Br2) gives vicinal dibromides (1,2-dibromides). Notes: The bromines add to opposite faces of the double bond (“anti addition”). Sometimes the solvent is mentioned in this reaction – a common solvent is carbon tetrachloride (CCl4).
What does h2 do to an alkene? ›Alkenes can be reduced to alkanes with H2 in the presence of metal catalysts such as Pt, Pd, Ni or Rh. The two new C-H σ bonds are formed simultaneously from H atoms absorbed into the metal surface. The reaction is stereospecific giving only the syn addition product.
What does propene and HBr gives? ›Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yields 1-bromopropane.
How do you write an electrophilic addition reaction of alkenes? ›An electrophile, H+, is generated from hydrogen bromide, which attacks the double bond to form a carbocation. Since secondary carbocation is more stable than primary carbocation, secondary carbocation predominates the formation of ions. Finally, Br– attacks the carbocation to form alkyl halides.
What are electrophilic additions to alkenes? ›Instead of a substitution, alkenes undergo electrophilic addition, a reaction in which a two-component reactant adds across the double bond. The reaction begins with an electrophilic attack by the double bond onto the reactant which produces a carbocation that then undergoes nucleophilic attack.
What is the order of electrophilic addition of alkenes? ›So, the decreasing order of EA is: (II) - (alkene) > I - (alkyne) > III - (alkene with one EWG) > (IV) - (alkene with two EWGs).
What is the addition reaction of alkenes? ›In an addition reaction an alkene adds elements to each of the carbons involved in the π-bond, resulting in formation of sp3 carbons from sp2 carbons. This is one of the most important types of reactions that alkenes undergo.
What is electrophilic addition of alkynes? ›Alkynes undergo electrophilic addition in much the same manner as alkenes, however, the presence to two pi bonds allows for the possibly of the addition happening twice. The addition of one equivalent of hydrogen chloride or hydrogen bromide converts alkynes to haloalkenes.
What are the three addition reactions of alkenes? ›Alkene molecules can undergo different types of addition reactions. Alkene molecules can undergo hydrogenation, hydrohalogenation, halogenation, and hydration-type addition reactions.
Why is it called electrophilic addition reaction? ›An electrophilic addition reaction is an addition reaction which happens because what we think of as the "important" molecule is attacked by an electrophile. The "important" molecule has a region of high electron density which is attacked by something carrying some degree of positive charge.
What are the 5 types of alkene reactions? ›
Alkanes can undergo five major types of reactions: (1) Combustion Reactions, (2) Addition Reactions, (3) Elimination Reactions, (4) Substitution Reactions, and (5) Rearrangement Reactions.
What are electrophilic reactions examples? ›In electrophilic aromatic substitution reactions, an atom attached to an aromatic ring is replaced with an electrophile. Examples of such reactions include aromatic nitrations, aromatic sulphonation, and Friedel-Crafts reactions.
Is electrophilic addition two step? ›In an electrophilic addition, proton abstraction occurs first, generating a positively-charged intermediate. Nucleophilic attack is the second step.
Do alkenes undergo electrophilic reaction? ›Thus, alkenes prefer to undergo electrophilic addition reactions while arenes prefers electrophilic substitution reactions.
Which reactions are most common in alkenes electrophilic substitution reactions? ›Oxidation and Polymerisation of Alkenes.