Hydrocarbon Reactions for the DAT
/Learn key DAT concepts related to hydrocarbon reactions of alkenes, alkynes, and aromatic compounds, plus practice questions and answers
Everything you need to know about hydrocarbon reactions for the dat
Table of Contents
Part 1: Introduction to hydrocarbons
Part 2: Fundamentals of hydrocarbon reactions
a) Transformations
b) Making alkenes
Part 3: Alkene reactions
a) Additions
b) Zaitsev’s rule
c) Markovnikov’s rule
d) Additions revisited
e) Hydrohalogenation
f) Hydrogenation
g) Hydroxylation
h) Cleavage
i) Epoxides
j) Product types
Part 4: Alkyne reactions
a) Making alkynes
b) Addition
c) Hydration
d) Reduction
e) Alkylation
f) Oxidation
Part 5: Aromatic reactions
a) EAS overview
b) Substituent effects
c) Halogenation
d) Nitration
e) Sulfonation
f) Friedel-Crafts
g) Side chain reactions
h) Reduction
i) Oxidation
j) Polysubstitution
k) Diazonium salts
Part 6: High-yield terms
Part 7: Questions and answers
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Part 1: Introduction to hydrocarbon reactions
Hydrocarbons are vital to organic chemistry and are the backbone for organic compounds. It is from this backbone, consisting of hydrogen and carbon, that other groups extend from. Hydrocarbons consist of interconnected carbon and hydrogen atoms, forming chain-like molecules. In this section, you will explore DAT-specific reactions and transformations between the different classes of hydrocarbon molecules. These include alkanes, alkenes, alkynes, and aromatic compounds. Test your knowledge with practice questions and answers at the end of this guide.
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Part 2: Fundamentals of hydrocarbon reactions
a) Transformations
A transformation reaction refers to any chemical reaction in which reactants undergo a change in molecular structure. This change manifests in the corresponding products. Transformation reactions encompass a wide range of chemical processes, including addition reactions, elimination reactions, substitution reactions, oxidation-reduction reactions, rearrangement reactions, and many others. Each type of transformation reaction involves specific changes to the molecular structure of the reactants to produce the desired products.
For example, addition reactions involve atoms or groups being incorporated into a molecule. This typically occurs across a double or triple bond, resulting in an increase in the number of substituents on the carbon atoms involved in the bond. In contrast, elimination reactions involve the removal of atoms or groups from a molecule, often resulting in the formation of a double bond or triple bond.
Substitution reactions are characterized by the replacement of one functional group or atom with another. Oxidation-reduction reactions involve the transfer of electrons between reactants, leading to changes in their oxidation states. Finally, rearrangement reactions simply rearrange atoms within a molecule to yield a different structural isomer.
For a more in-depth discussion of these reactions, see our organic chemistry guide for substitution and elimination reactions and our general chemistry guide for oxidation-reduction.
b) Making alkenes
A classic reaction converting an alkane into an alkene involves the elimination of a water molecule, termed a dehydration reaction. A prerequisite for this reaction is the presence of an alcohol group on the alkane. One of the commonly used methods to achieve this conversion is through catalytic dehydrogenation, often employing a metal catalyst such as platinum (Pt), palladium (Pd), or nickel (Ni).
In catalytic dehydrogenation, heat is applied to the alkane in the presence of the catalyst. The catalyst facilitates the removal of two hydrogen atoms from adjacent carbon atoms in the alkane molecule, resulting in the formation of a carbon-carbon double bond (alkene) and hydrogen gas as a byproduct. This process, called β-elimination, involves the breaking of adjacent bonds and the resulting formation of a new pi bond.
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Part 3: Alkene reactions
Alkene reactions are diverse, including a broad range of transformation reactions. Despite their variety, many share several common features:
Electrophilic addition: Alkenes typically undergo addition reactions, where an electrophile (electron-deficient chemical species) adds to the π-bond of the alkene, breaking it and forming new σ-bonds.
Formation of carbocations: Many alkene reactions involve the formation of carbocations as intermediates. In this process, the electrophile attacks the alkene, leading to the formation of a carbocation intermediate. This intermediate then reacts with a nucleophile to yield the final product.
Regioselectivity: The regioselectivity of alkene reactions is often governed by Markovnikov's rule, which states that in the addition of a protic acid (HX) to an alkene, the hydrogen atom bonds to the carbon atom with more hydrogen substituents. Anti-Markovnikov additions can also occur under certain conditions.
Stereochemistry: Alkene reactions can lead to the formation of stereoisomers, especially when asymmetric carbon atoms are involved or when the reaction proceeds through a cyclic transition state.
While each specific reaction has its nuances, these common characteristics provide a framework for understanding and predicting alkene reactivity. A table including a comprehensive list of reactions is given at the bottom of the alkene section for reference.
a) Addition
In an alkene addition, the double bond of an alkene is broken and new bonds are formed with the addition of groups to the carbon atoms of the initial double bond. In these reactions, the double bond acts as a nucleophile (meaning it is prone to initiating interaction with atoms and molecules containing an electron deficiency), attacking electrophiles (electron-deficient species) that are presented to it. As a result of these reactions, the number of substituents attached to the carbons in the initial double bond increases. The two most fundamental addition reactions on the DAT are halogen addition and OH addition. Relevant mechanisms are also covered.
A halogen addition, or halogenation, involves the breaking of a double bond and the joining of a halogen (X) atom. These halogen atoms are most commonly Br and Cl. First, the nucleophilic C-C double bond attacks one of the halogens (X) in an diatomic halogen (X2) molecule. The X that was attacked forms a bond with both carbons. The lone pair from the initial X-X bond moves to the isolated X, resulting in a nucleophilic X- atom. Because of the electronegativity of halogens, both carbons bonded to X are electrophilic. The remaining X-nucleophile attacks one of these electronegative carbon atoms. This breaks the bond between that C and the initial X that was attacked by the alkene. The end result to this reaction is the attachment of a halogen atom on both carbon atoms that were initially connected by a double bond. Regarding stereochemistry, the product is anti as the X’s are trans to one another.
FIGURE 1: HALOGENATION REACTION MECHANISM
Alcohol (OH) groups can be added to alkenes via hydration reactions. This process is started with the association of an alkene and an OH group (from a compound like water or hydronium). The double bond in the alkene attacks the hydrogen atom of a hydronium ion (H3O+). Then, the partially negative oxygen atom of the resulting H2O group is attracted to the positively charged carbon atoms in the alkene. The nucleophilic double bond attacks the partially positive hydrogen atom on the H2O group. As a result, one of the carbon atoms in the alkene forms a bond with the oxygen atom from the H2O group, while the other carbon atom retains its bond with the rest of the alkene molecule. This process breaks the pi bond in the alkene and forms a new bond between one of the carbon atoms and the oxygen atom from the OH group. Finally, a separate water molecule removes a hydrogen atom from the attached H2O group.
FIGURE 2: HYDRATION REACTION MECHANISM
Note that many reagents are capable of hydrating alkenes. While the mechanism above is specific to hydration with H3O+, different mechanisms and product orientations are possible. These are specific to the reagent used, and H3O+ is the most common on the DAT.
b) Zaitsev’s rule
When the final product of a reaction is an alkene and more than one product is possible, it is important to differentiate the major (more favorable) and minor (less favorable) products. Note that the former has a more thermodynamically stable double bond than the latter. This method of determining regioselectivity is called Zaitsev’s rule.
Alkene stability is determined by the substitution of possible alkene products. In this context, substitution directly relates to the number of R groups attached to an alkene. For instance, a trisubstituted alkene has 3 R group substituents, whereas a disubstituted alkene has 2. If these were both products of the same reaction, using Zaitsev’s rule, it can be inferred that the tetrasubstituted alkene is the major product, while the disubstituted one is minor.
FIGURE 3: ZAITSEV’S RULE
c) Markovnikov’s rule
Another common organic chemistry reaction is hydrohalogenation, or the addition of a protic acid (HX) to an alkene. As with the previous addition reactions, the reaction is initiated by the nucleophilicity of the alkene. Shown in the reaction below, halogenation results in both the hydrogen and halogen atoms being added to the alkene. To determine whether the H or the X attaches to the more substituted C, stability is again considered, this time with Markovnikov’s rule. In the instance below, the standard addition of HBr to the given alkene will favor the product with Br being more substituted. An easy way to remember this is that hydrogens prefer to be with other hydrogens. The more substituted product is termed the Markovnikov product.
An exception to Markovnikov’s rule is that, given certain reagents, the “anti-Markovnikov”, or less substituted, product is favored. With hydrohalogenation, the standard Markovnikov reaction involves HBr as the only reagent, whereas including a peroxide (ROOR) and heat as reagents favors the anti-Markovnikov product.
FIGURE 4: HYDROHALOGENATION PRODUCTS
d) Additions revisited
By considering the rules of Markovnikov and Zaitsev, you can better understand many alkene reactions, including addition reactions. Covered above, alkene hydration with H3O+ (acid-catalyzed hydration) favors the Markovnikov product. In the initial stage of this reaction, the acid catalyst protonates the double bond, generating a carbocation intermediate. This carbocation moves to its most stable position, often through the shifting of hydride. Again, stability is determined by the number of R group substituents attached to the carbocation.
FIGURE 5: HYDRIDE SHIFT FOR CARBOCATION STABILITY
Another hydration reaction producing a Markovnikov product is the oxymercuration-demercuration reaction. To hydrate an alkene in an anti-Markovnikov manner, the hydroboration-oxidation reaction is used. In addition to this being an anti-Markovnikov addition, H and OH groups are added with syn-stereochemistry.
e) Hydrohalogenation
Another addition reaction is hydrohalogenation. The difference between this and the previously covered halogen addition is that hydrohalogenation involves the addition of HX. In this reaction mechanism, an alkene attacks the hydrogen of an HX, causing carbocation formation on the initial alkene. Hydrogen is added to the less substituted side of the alkene, while the carbocation is present on the more substituted side. The carbocation is rearranged if higher stability is possible. Finally, the remaining X- nucleophile attacks this carbocation, attaching to the associated carbon.
FIGURE 6: HYDROHALOGENATION MECHANISM
f) Hydrogenation
Up to this point, we’ve covered the addition of several functional groups to alkenes, often with hydrogen also being added. Hydrogenation is an example of a reaction in which solely hydrogens are added to an alkene. It's thermodynamically favorable to hydrogenate an alkene; however, this reaction only occurs with the help of a catalyst, often Pd/C. While the mechanism for this reaction is not assessed on the DAT, it should be known that the metal catalyst causes a syn addition to occur.
FIGURE 7: HYDROGENATION REACTION
g) Hydroxylation
From beginning to end, a hydroxylation reaction involves converting an alkene into a diol, meaning an OH group is attached to both carbons of the alkene. 2 types of diols, syn and anti, can be formed. Their respective formation depends on the reagents used. OsO4/Pyridine or NaHSO3 results in syn diol formation. Cold, dilute KMnO4 can also be used as a reagent to form a syn diol. When treated with mCPBA and subsequently exposed to hydroxide (often NaOH/H2O), an anti diol is formed. With anti-diol formation, H3O+ can be used instead of hydroxide.
FIGURE 8: HYDROXYLATION REACTION
h) Cleavage
In an alkene cleavage reaction, a double bond is cleaved in an oxidation reaction. Two variables determine the product’s identity: reagents used and whether the alkene is internal or terminal. Some reagents cause a gentle, 2-step cleavage, whereas others result in a strong, 1-step cleavage. For gentle cleavage, oxidation converts internal carbons into ketones and terminal carbons into aldehydes. For crude cleavage, oxidation converts internal carbons into ketones and terminal carbons into carboxylic acids. A visual displaying the products is shown below. You can find the reagents for gentle and crude cleavages in the table at the bottom of this section.
FIGURE 9: ALKENE CLEAVAGE
i) Epoxides
Another alkene reaction you may see on the DAT involves the formation of an epoxide. The mechanism for this is complex and not pertinent for your exam. For this reaction, be able to recall that the reagent is peroxyacid (RCO3H) or mCPBA. Also, examine the visual below to familiarize yourself with the structure of an epoxide.
FIGURE 10: EPOXIDE FORMATION
j) Product types
When you’re considering the potential products of an alkyne reaction, it is important to be able to distinguish between kinetic and thermodynamic products. Before continuing, consider reviewing your general chemistry energy diagrams.
The differentiation between kinetic and thermodynamic products is relevant when a strong acid is added to a diene. In this instance, the resulting carbocation intermediate can generate either an external or internal alkene product. The external alkene (kinetic product) is less stable, but the transition state required to reach it requires a smaller energy input. On the other hand, the internal alkene (thermodynamic product) is more stable, but its energy state calls for more energy.
FIGURE 11: THERMODYNAMIC VS KINETIC PRODUCTS
For review, remember that kinetic products form faster, are favored at lower temperatures, have higher carbocation stability, and have a more external double bond at the end. Thermodynamic products take longer to form, are favored at higher temperatures (due to greater energy input), and have a more internal double bond at the end.
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-O3, followed by reduction (e.g., Zn/H2O or DMS) -OsO4/H2O2, HIO4 |
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-O3, H2O2 |
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