Name: counting rings and double bonds in molecules
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 5 s
Channel: JoVE

Alkanes contain the maximum possible number of hydrogens, 2n + 2, where n is the number of carbon atoms. They are called saturated hydrocarbons.
Hydrocarbons with multiple bonds are unsaturated and have fewer hydrogens per carbon atom than their saturated analogs. In cyclic compounds, the presence of every ring costs two hydrogens.
A compound possesses one degree of unsaturation for every &#960; bond and ring present in the molecule, which corresponds to each pair of missing hydrogen atoms in hydrocarbons.
In such compounds, the degree of unsaturation, also called the index of hydrogen deficiency, or saturation number, is half the difference between the maximum possible and actual number of hydrogens.
If a molecular formula is C4H6, the compound has two degrees of unsaturation. The molecule may have one ring and one double bond or two double bonds. Other possibilities are the presence of two rings or one triple bond.
The degree of unsaturation can also be calculated for ions and functionalized hydrocarbons by adjusting for the differences in valency.
In organohalogen compounds, the monovalent halogens are considered as equivalent to hydrogen. As such, the degrees of unsaturation of 1-br

degree of unsaturation

Degree of Unsaturation

The degree of unsaturation (U), or index of hydrogen deficiency (IHD), is defined as the difference in the number of pairs of hydrogen atoms between the compound and the acyclic alkane with the same number of carbon atoms. Each double bond or ring costs two hydrogen atoms compared to a saturated analog and results in one degree of unsaturation.
The degree of unsaturation for hydrocarbons is U = (2C + 2 &#8722; H) / 2, where C is the number of carbon atoms and H is the number of hydrogen atoms.
For substituted hydrocarbons and ions, the degree of unsaturation is calculated by adjusting the equation for the difference in valency.
In organohalogen compounds, the halogen atoms, being monovalent, are treated as equivalent to hydrogen atoms. Thus, the formula to calculate the degree of unsaturation for compounds containing halogen atoms is U&#160;=&#160;(2C&#160;+&#160;2&#160;&#8722;&#160;(H&#160;+&#160;X))&#160;/&#160;2, where X is the number of halogen atoms.
Divalent oxygen atoms in organooxygen compounds are ignored while calculating the saturation number. Each trivalent nitrogen atom in organonitrogen compounds adds to the numerator: U&#160;=&#160;(2C&#160;+&#160;2&#160;+&#160;N&#160;&#8722;&#160;H)&#160;/&#160;2, where N is the number of nitrogen atoms.
Accordingly, the overall formula for degree of unsaturation is U = (2C + 2 + N &#8722; (H + X)) / 2.

The degree of unsaturation (U), or index of hydrogen deficiency (IHD), is defined as the difference in the number of pairs of hydrogen atoms between the ...

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Organic Chemistry

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Alkene Structure and Reactivity

KEY TERMS AND CONCEPTS

counting rings and double bonds in molecules

Degree of unsaturation helps count rings and double bonds in an organic molecule. It is also called the index of hydrogen deficiency, or IHD. The idea is based on comparing a compound with the acyclic alkane that has the same number of carbon atoms.

Each double bond or ring removes two hydrogen atoms compared with a saturated analog. That loss equals one degree of unsaturation. For hydrocarbons, the formula is U = (2C + 2 - H) / 2, where C is the number of carbon atoms and H is the number of hydrogen atoms.

For substituted hydrocarbons and ions, the calculation must be adjusted for valency differences. In organohalogen compounds, halogen atoms are monovalent, so they are treated like hydrogen atoms. The formula becomes U = (2C + 2 - (H + X)) / 2, where X is the number of halogen atoms.

Organooxygen compounds are handled differently because oxygen is divalent, so it is ignored in the saturation count. In organonitrogen compounds, each trivalent nitrogen atom adds to the numerator. The formula is U = (2C + 2 + N - H) / 2, where N is the number of nitrogen atoms. For compounds that contain nitrogen and halogens, the overall formula is U = (2C + 2 + N - (H + X)) / 2.

Counting Rings and Double Bonds in Molecules

counting-rings-and-double-bonds-in-molecules

regioselectivity of electrophilic additions to alkenes: markovnikov's rule

regioselectivity-electrophilic-additions-to-alkenes-markovnikov-s

Markovnikov Addition Paths in Alkenes

Markovnikov addition in alkenes explains why one product often forms in greater amount during hydrohalogenation. When an unsymmetrical alkene reacts, two haloalkane constitutional isomers are possible. In most cases, hydrogen adds to the vinylic carbon with more hydrogens, and the halide part adds to the more substituted carbon. This preference is called regioselectivity. Regioselective reactions favor bond formation at one site over another, so one constitutional isomer becomes the major...

structure and bonding of alkenes

Alkene Bonding, Properties, and Uses

Alkenes are unsaturated hydrocarbons with one or more carbon-carbon double bonds. They are also called olefins, and they are broadly divided into alkenes and cycloalkenes. The general formula for an alkene is CnH2n. The carbons in a double bond are sp2 hybridized. This gives each carbon a trigonal planar shape. The double bond has two parts: a sigma bond from overlap of hybrid orbitals and a pi bond from side-by-side overlap of unhybridized 2p orbitals. Each carbon also bonds to two hydrogen...

nomenclature of alkenes

IUPAC Rules for Alkene Naming

IUPAC rules for alkene naming show how to name carbon chains with a double bond. A simple alkene changes the alkane suffix -ane to -ene. The double bond must be part of the parent chain, which is the longest carbon chain that contains the maximum number of double bonds. The chain is numbered so the doubly bonded carbon atoms get the lowest possible numbers. The location of the double bond is given by the number of its first carbon atom. In branched alkenes, the double bond takes priority over...

Degree of unsaturation helps count rings and double bonds in an organic molecule. It is also called the index of hydrogen deficiency, or IHD. The idea is based on comparing a compound with the acyclic alkane that has the same number of carbon atoms. Each double bond or ring removes two hydrogen atoms compared with a saturated analog. That loss equals one degree of unsaturation. For hydrocarbons, the formula is U = (2C + 2 - H) / 2, where C is the number of carbon atoms and H is the number of...

isomerism in alkenes

Cis, Trans, and E/Z Alkene Forms

Alkene isomers can differ in both bond position and spatial arrangement. 1-butene and 2-butene are constitutional isomers because the double bond is in a different place. 2-butene also shows stereoisomerism, which means its atoms are connected the same way but arranged differently in space. In 2-butene, the two methyl groups can sit on the same side of the double bond or on opposite sides. When the methyl groups are on the same side, the compound is called cis-2-butene. When they are on...

relative stabilities of alkenes

Alkene Stability: Heat and Structure

Alkene stability depends on heats of hydrogenation and on the structure around the double bond. A lower heat of hydrogenation means the alkene is more stable. Three main factors help explain these differences: the number of substituents on the double-bond carbons, hyperconjugation, and double-bond stereochemistry. Substitution across the double bond has a strong effect on stability. An alkene with two smaller substituents is more stable than its isomer with one large substituent. For example,...

introduction to electrophilic addition reactions of alkenes

introduction-to-electrophilic-addition-reactions-of-alkenes

Alkene Addition Reactions and Redox Changes

Alkene addition reactions show how a carbon-carbon double bond reacts with electrophiles and changes during the process. A simple, unconjugated alkene has a double bond with high electron density, so it can act as a weak base or a nucleophile. Its filled π orbital, called the HOMO, can interact with the empty LUMO of an electrophile.

key terms and concepts

Learn how degree of unsaturation helps count rings and double bonds, and how to adjust the formula for halogens, oxygen, and nitrogen in organic compounds.

Counting Rings and Double Bonds in Molecules | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Counting Rings and Double Bonds in Molecules

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