8.3 Nomenclature and Stability of Alkenes

What are the four basic steps for naming alkanes and alkyl halides?

1. Identify the parent chain

2. Identify the substituents

3. Number the substituents

4. Arrange the substituents alphabetically

Are the same four steps used to name alkenes?

Yes, with a few additional rules

What change is made to the parent name when naming an alkene?

Replace “-ane” with “-ene”

What does the ending “-ene” indicate?

The compound contains a C=C double bond

What is the parent name for an alkane with five carbons?

Pentane

What is the parent name for an alkene with five carbons?

Pentene

How do you choose the parent chain when naming alkanes?

Choose the longest continuous carbon chain; if there’s a tie, choose the one with more substituents

What additonal rule applies when choosing the parent chain for an alkene?

The parent chain must include the C=C (π bond)

What do you do if two chains are the same length?

Choose the chain that contains the double bond as the parent

Why must the parent chain include the double bond?

Because the double bond defines the compound as an alkene, not an alkane

What is the parent name of a 6-carbon alkane?

Hexane

What is the parent name of a 6-carbon alkene with a double bond?

Hexene

In the given example, why is “hexene” the correct parent?

Because the chosen chain includes π bond, even though another chain without the double bond is the same length

What gets top priority when numbering the parent chain of an alkene?

The π bond (double bond)

How should the parent chain of an alkene be numbered?

From the end closest to the double bond, giving it the lowest possible number

Why is the π bond given the lowest number?

Because the location of the double bond is the most important feature in naming an alkene

What makes the left example correct?

Numbering 1 → 2 → 3 reaches the double bond first, giving it the lowest locant

What makes the right example incorrect?

Numbering from the opposite end gives the double bond a higher number, which is not allowed

What is the general rule for numbering alkenes?

Always number toward the double bond

How do you indicate the position of the double bond in an alkene?

Use a single number showing where the π bond starts

If a double bond is between carbons 2 and 3, which number is used?

2 (because the double bond starts at carbon 2)

Where can the double-bond position number be placed in the name?

1. Before the parent name (1979 IUPAC)

2. Before “-ene” (1993 and later IUPAC)

Both are correct

Examples:

• 5,5,6-trimethyl-2-heptene

• 5,5,6-trimethylhept-2-ene

What does the double-bond locant tell you?

The carbon number where the double bond begins

Are both naming formats acceptable on exams?

Yes- both are officially recognized IUPAC names

What does the E configuration mean for a double bond?

The two highest-priority groups on each carbon of the double bond are on opposite sides

What does the Z configuration mean?

The two highest-priority groups are on the same side of the double bond

How do you determine priority around a double bond?

Using the Cahn-Ingold-Prelog (CIP) priority rules

Where is the E/Z stereo descriptor placed in the name?

At the very beginning of the name, in parentheses

Example: What does (E)-5,5,6-trimethyl-2-heptene tell you?

• The double bond starts at carbon 2

• The higher-priority groups are on opposite sides → (E)

• Substituents are on carbons 5, 5, and 6

Why is the stereo descriptor required?

Because E and Z alkenes are different molecules with different shapes and properties

In a cyclic alkene, which carbons automatically get numbers 1 and 2?

The two carbons of the double bond

Do you need to specify the double bond position in simple cyclic alkenes?

No. The double bond is always between C1 and C2

What is the correct parent name for a six-membered ring with a double bond?

Cyclohexene, not 1-cyclohexene

After assigning C1 and C2 to the double bond, how do you number the rest of the ring?

Number in the direction that gives substituents the lowest possible numbers

Why is “3,3-dibromocyclohexene” the correct name in the example?

Because numbering from the double bond toward the substituents givens them the lowest locants (3 and 3)

What makes the incorrect examples wrong?

They number in the wrong direction, giving substituents higher and incorrect numbers (like 1,2 or 1,3)

What is the general rule for substituent numbering in cyclic alkenes?

Always start at the double bond (C1-C2) and number toward the nearest substituent

What is a compound with two double bonds called?

A diene

How do you indicate the positions of double bonds in a diene?

Use two numbers, each showing where a double bond starts.

Example: (2Z, 4E)-heptadiene

Where can the numbers for the double bonds be placed?

Either before the parent name (2,4-heptadiene) or before “-diene” (hepta-2,4-diene.) Both are correct.

What is a 10-carbon chain with three double bonds called?

Decatriene

What is an 11-carbon chain with four double bonds called?

Undecatetraene

What does “triene” indicate?

Three double bonds

What does “tetraene” indicate?

Four double bonds

What is the general naming pattern for polyenes?

Parent name + “di-/tri-/tetra-ene”, with numbers showing the position of each C=C bond

What is the common name for ethene (CH2=CH2)?

Ethylene

What is the common name for propene (CH2=CH-CH3)?

Propylene

What is stryrene?

A benzene ring attached to a vinyl group (C6H5-CH=CH2)

Are ethylene, propylene, and styrene official IUPAC names?

No- they are common names that IUPAC still recognizes due to widespread industrial use

Why are these common names important to know?

They appear frequently in industry, polymer chemistry, and exam problems

What is the mehylene group?

=CH2 as a substituent

What is the vinyl group?

-CH=CH2 attached to a parent chain

What is the allyl group?

-CH2-CH=CH2 attached to a parent chain

Are these names systemic or common names?

They are common names that IUPAC allows when these groups appear as substituents

Why are these names important?

They show up frequently in synthesis, reactivity, and naming problems

Which substituent contains a double bond directly attached to the parent chain?

The vinyl group

Which substituent has the double bond one carbon away from the parent chain?

The allyl group

What does “degree of substitution” mean for an alkene?

The number of alkyl groups attached directly to the double-bonded carbons

What is a mono-substituted alkene?

The double bond has one alkyl substituent (least stable)

What is disubstituted alkene?

The double bond has two alkyl substituents

What is trisubstituted alkene?

The double bond has three alkyl substituents

What is a tetra-substituted alkene?

The double bond has four alkyl substituents (most stable)

Does “substitution” here refer to substitution reactions from Chapter 7 ?

No. It refers only to how many alkyl groups are bonded to the double bond

Why does more substitution increase stability?

More alkyl groups stabilize the double bond through hyper conjugation and electron donation

What is the main factor that increases alkene stability?

The number of alkyl group attached to the double bond (degree of substitution)

Which is more stable: a more substituted or less substituted alkene?

A more substituted alkene

Why do alkyl groups increase alkene stability?

They donate electron density via hyperconjugation, stabilizing the double bond

Which is more stable: cis or trans alkenes?

Trans alkenes

Why are cis alkenes less stable?

They have steric strain, because bulky groups are on the same side of the double bond

What type of strain do cis alkenes experience?

Steric strain (repulsion from groups being too close)

What is the overall stability order cis/trans isomers?

Trans> Cis

Which cycloalkenes cannot have a trans double bond?

Rings with fewer than 7 carbons (cyclopropene → cyclohexene)

What configuration must small cycloalkenes have?

Cis only (trans is impossible)

Why can small rings only have cis double bonds?

The ring is too small to twist into the geometry needed for a trans π bond without severe strain

Why is the configuration not written in names like “cyclohexene”?

The cis configuration is automatically inferred, so no E/Z or cis/trans label is needed

Can cycloheptene have a trans double bond?

In theory yes, but trans-cycloheptene is unstable at room temperature

What is the smallest ring that can stably contain a trans double bond?

Cyclooctene (8-membered ring)

Why can cyclooctene handle a trans double bond?

The ring is large and flexible enough to reduce the strain of opposite-side substituentsThe ring is large and flexible enough to reduce the strain of opposite-side substituents.

What is Bredt’s Rule?

A double bond cannot be placed at a bridgehead position in a small bicyclic system because it will be unstable

What is a bridgehead carbon?

A carbon atom shared by two or more rings in a bicyclic structure

Why is a bridgehead double bond unstable?

Because the bridgehead carbon cannot achieve the required planar geometry for a π bond

What does the red double bond in the example represent?

A trans double bond at a bridgehead- an arrangement that violates Bredt’s Rule

Does Bredt’s Rule apply to all bicyclic systems?

It applies to small bicyclic systems; only large ring systems can support a stable bridgehead alkene

What is required for a stable bridgehead double bond?

A large enough bicyclic system that allows the double-bond carbon atoms to become planar