Understanding Alkenes: Structure, Reactivity, Unsaturation, Naming, and Addition, Slides of Chemistry

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6. Alkenes: Structure

and Reactivity

Based on McMurry’s Organic Chemistry, 7 th edition

Alkene - Hydrocarbon With

Carbon-Carbon Double Bond

 Also called an olefin but alkene is better

 Includes many naturally occurring materials  Flavors, fragrances, vitamins

6.1 Industrial Preparation and Use of Alkenes  Ethylene and propylene are the most important organic chemicals produced  Ethylene, propylene and butene are made by thermal cracking of light alkanes (petroleum)

6.2 Calculating Degree of Unsaturation  Relates molecular formula to possible structures  Degree of unsaturation : number of multiple bonds or rings  Formula for a saturated acyclic compound is CnH 2 n+ 2  Each ring or multiple bond replaces 2 H's

Degree of Unsaturation With

Other Elements

 Organohalogens (X: F, Cl, Br, I)  Halogen replaces hydrogen  C 4

H

6 Br 2 and C 4

H

8 have one degree of unsaturation  Organoxygen compounds (C,H,O) - if connected by single bonds  These don't affect the total count of H's

Organonitrogen compounds  Nitrogen has three bonds  So if it connects where H was, it adds a connection point  Subtract one H for equivalent degree of unsaturation in hydrocarbon

6.3 Naming of Alkenes  Name the parent hydrocarbon—change ending to “–ene”  Number carbons in chain so that double bond carbons have lowest possible numbers  Rings have “ cyclo ” prefix—double bond always C# 1 , C# 2  Multiple “C=C“ are named as “diene” “triene” “tetraene” etc…  Alkenes higher priority than alkanes: even shorter chain

Many Alkenes Are Known by Common Names

Rotation of  Bond Is Prohibitive  This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond).  Creates possible alternative structures

 The presence of a carbon-carbon double bond can create two possible structures

 cis isomer - two similar groups on same side of the

double bond

 trans isomer - similar groups on opposite sides

 Each carbon must have two different groups for these isomers to occur

6.5 Naming Alkenes Using E and Z isomers Naming Alkenes Using E and Z isomers

Ranking Naming Alkenes Using E and Z isomers Rules RULE 1  Consider the atomic number of the atoms bonded directly

to a specific sp

2 carbon  Higher atomic number gets higher priority  Br > Cl > S > P > O > N > C > H

RULE 2

 If atomic numbers are the same, compare at next connection point at same distance  Compare until something has higher atomic number  Do not combine – always compare Extended Comparison

RULE 3

 Substituent is drawn with connections shown and no double or triple bonds  Added atoms are valued with 0 ligands themselves Dealing With Multiple Bonds: