Monosubstituted alkenes are alkenes with one substituent attached to the double bond, usually one alkyl group. In Organic Chemistry, that substitution pattern affects stability and how the alkene reacts in hydroboration-oxidation.
Monosubstituted alkenes are alkenes where only one carbon of the carbon-carbon double bond has an attached substituent. The other double-bond carbon is usually a hydrogen-rich, less substituted carbon. In practice, this means the alkene has one alkyl group total on the double bond, so it sits just above ethene and below more substituted alkenes in stability.
In Organic Chemistry, substitution is not just a naming detail. It changes how the pi bond behaves. Alkyl groups push electron density toward the double bond through hyperconjugation and weak inductive donation, which helps stabilize the alkene. That is why a monosubstituted alkene is more stable than an unsubstituted one, but less stable than a disubstituted or trisubstituted alkene.
You will often see monosubstituted alkenes in simple molecules like propene or in ring systems where only one alkyl group is attached to the double bond. A structure like 2-methylpropene is more substituted overall, so it is not a monosubstituted example, which makes it a useful comparison when you are practicing alkene classification. The easiest way to count is to ignore the double bond itself and count carbon groups attached directly to the two sp2 carbons.
Their stability matters because alkene substitution affects reaction outcomes. In hydroboration-oxidation, the reagent adds across the double bond in an anti-Markovnikov way, so the boron ends up on the less substituted carbon first, and then oxidation turns that position into an alcohol. For a monosubstituted alkene, that usually means the final alcohol places the hydroxyl group on the less substituted side of the original double bond.
One common mistake is mixing up stability with reactivity in the same direction. More substituted alkenes are usually more stable, but that does not mean they always react the same way or give the same product placement. In a problem set, the term often shows up when you compare alkene isomers, rank heats of hydrogenation, or predict where hydroboration-oxidation puts the OH group.
Monosubstituted alkenes matter because substitution is one of the first things you check when comparing alkene stability. If you are given several alkene isomers, the substitution pattern often tells you which one is more stable and which one has the lower heat of hydrogenation. That shows up in reaction problems where the more stable alkene is favored at equilibrium or where elimination can make more than one possible alkene.
They also give you a clean way to think about regiochemistry. In hydroboration-oxidation, the reagent adds so that the boron ends up on the less substituted carbon, which later becomes the alcohol. A monosubstituted alkene makes that pattern easier to see because one side of the double bond is clearly less crowded.
This term also trains you to read structures fast. Instead of memorizing every alkene name, you can count how many carbon substituents are attached to the double bond and use that to predict relative stability, product placement, and likely exam answers. It is a small label, but it unlocks a lot of mechanism-based reasoning.
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Visual cheatsheet
view galleryAlkenes
Monosubstituted alkenes are one subclass of alkenes, so you still need the basic alkene picture first: a carbon-carbon double bond with pi reactivity. When you identify a monosubstituted alkene, you are narrowing down how that double bond is substituted and how it will behave in addition reactions.
Stability
Substitution changes alkene stability, and monosubstituted alkenes sit near the lower end of the stability scale. They are more stable than unsubstituted ethene because an alkyl group can donate electron density, but they are less stable than alkenes with more carbon groups attached to the double bond.
Hydroboration-Oxidation
This reaction often appears with monosubstituted alkenes because the regiochemistry is easy to track. Boron adds to the less substituted carbon, and oxidation replaces that C-B bond with C-OH. If you know the alkene is monosubstituted, you can predict the alcohol product more confidently.
Hyperconjugation
Hyperconjugation is one reason alkyl substitution stabilizes alkenes. A monosubstituted alkene has some of that stabilization, but not as much as a more substituted alkene with more adjacent C-H and C-C bonds available for electron donation into the pi system.
A quiz question might show three alkene structures and ask you to rank them by stability or identify the monosubstituted one. You solve it by counting how many carbon groups are directly attached to the double bond carbons, then comparing substitution levels. In reaction problems, the term shows up when you predict the product of hydroboration-oxidation and place the OH group on the less substituted carbon. If the question asks for a rationale, you can mention reduced steric crowding and hyperconjugative stabilization rather than just naming the product. On mechanism worksheets, you may also be asked to explain why a monosubstituted alkene is less stable than a disubstituted one but more stable than an unsubstituted alkene.
These are easy to mix up because both terms describe alkene substitution, but the count is different. A monosubstituted alkene has one carbon substituent attached to the double bond, while a disubstituted alkene has two. That difference changes stability, heats of hydrogenation, and often how fast an alkene reacts in addition problems.
A monosubstituted alkene has one carbon substituent attached to its double bond.
In Organic Chemistry, substitution matters because it changes alkene stability through hyperconjugation and electron donation.
Monosubstituted alkenes are more stable than unsubstituted alkenes, but less stable than more substituted alkenes.
In hydroboration-oxidation, a monosubstituted alkene still follows anti-Markovnikov addition, so the OH ends up on the less substituted carbon.
The fastest way to identify one is to count the carbon groups attached directly to the two sp2 carbons.
Monosubstituted alkenes are alkenes with one substituent attached to the double bond, usually one alkyl group. That substitution makes them more stable than unsubstituted alkenes, but not as stable as disubstituted or trisubstituted alkenes.
Look only at the two carbons in the double bond and count the carbon groups attached directly to them. If there is one carbon substituent total, it is monosubstituted. Hydrogens do not count toward substitution.
An alkyl substituent can donate electron density toward the double bond and stabilize it through hyperconjugation and weak inductive effects. That extra stabilization lowers the energy of the alkene compared with ethene.
They give an anti-Markovnikov alcohol, meaning the OH ends up on the less substituted carbon of the original double bond. The reaction is also syn overall, so both additions happen to the same face of the alkene.