Chapter 7- Alkenes

unsaturated

alkenes are _______ hydrocarbons

pi and sigma

alkenes have one ____ and one ____ bond

one double bond

alkenes must have atleast _________

sp²

alkenes are _____ hybridized

trigional planar, 120

alkenes have ____ _____ shape and ________ angles

CnH2n

alkene general formula is ________ (same as cycloalkanes so it will be specified)

stronger

double bond is ______ than a single covalent bond (not two times tho)

shorter

double bond is ______ than single bonds

stable, reactive

alkenes are relatively _____ and more ___ than alkanes

1) count number of carbons in longest cont. chain with the C=C bond, identify alkane and change to “ene” → pentane becomes pentene

2) identify location of double bond using lowest numbering system (in addition, if the double bond is between carbons 2 and 3, use 2 (lower of the two numbers)

3) chain is numbered end nearest the double bond

4) if there is a branched on it, alphabetically name it but still number from end closest to double bond

alkene nomenclature

diene or triene and locate the number of each double bond

if there is more than one double bond, use

-ylene

(ie: methane → methylene)

alkenes as substituent groups (alkenyl groups) remove “ane) and add

MEMORIZE

two alkenes as substitutent groups to memeorize

• Ethenyl → vinyl (CH2=CH)

• Propenyls → allyl (CH2=CH-CH2-)

• trigonal planar (sheet of paper image where C=C is in the paper)

alkene stereochemistry

rotation

due to the double bond- there are restrictions about the _____ → position is significant

• rotation must be restricted in molecule

• must be 2 nonidentical groups on each doubly bonded carbon atom (if one C has 2 identical groups it is not allowed)

cis/trans isomerism requirements

there are more than 2 groups at the end of each double bond (ie→ 4 different groups are attatched to the C=C bond)

E/Z isomerization is used over cis/trans when

• rank the 2 groups on each carbon using Cahn-Ingold priority (size based → atomic number directly attatched to the C)

E/Z usage

Z

if both high priority groups are on the same side

E

if priority groups are on opposite sides

monosubstitued (least stable) ———> tetra substitued (most stable) (ie: more alkyl groups across double bond the more stable)

alkene stability

elimination reactions

how are alkenes made generally?

a pair of atoms or group of atoms are removed from a molecule through acids, bases, or temps

what is an elimination reaction

• Unimolecular

• Yes carbocation (3 > 2> 1)

• No strong base

E1 reactions are

• ___molecular

• yes or no carbocation

• yes or no strong bases

• bimolecular

• no carbocation (bumps H to form double bond)

• strong base needed

E2 reactions are

• ___molecular

• yes or no carbocation

• yes or no strong bases

dehydration of alcohol

first way to yield an alkene

can be E1 or E2

H2O is lost and double bond forms

Utilizes strong acid

dehydration of alcohol

E2

primary alcohols undergo

E1

tertiary alcohols undergo

E1 or E2

secondary alcohols undergo

adjacent (from the beta carbon never the alpha (the one bearing the group))

an _____ hydrogen is always attacked to form the double bond

carbocation rearrangements

E1 can still have

mono (least stable) ——> tetra (most stable) where the more carbons the C=C is attached the more stable

alkene stability

elimination will occur such that a hydrogen is removed from the B carbon with fewer hydrogens (alpha is attatched to the leaving group while the B carbon are all carbons attatched to the alpha)

zaitseves rule

one reaction site is preferred over another (one product (major) is preferred over the other (minor))

regioselectivity

dehydrohalogenation of an alkyl halide

another way to make an alkene

elimination reaction that removes a hydrogen hallide from a substrate

dehydrohalogenation of an alkyl halide

halide

dehydrohalogenation of an alkyl halide results in a loss of

NaOH, KOH, or alkoxides (RO-) usually use CH3O-

strong base examples

• weak base

• polar solvent

• 3 degree

• yes carbocation

E1 dehydrohalogenation of an alkyl halide

• strong or weak base

• type of solvent

• type of alkyl halide

• yes or no carbocation

• strong base

• primary alkyl halide

• no carbocation

E2 dehydrohalogenation of an alkyl halide

• strong or weak base

• type of alkyl halide

• yes or no carbocation

REMEMEMEEMEMEBER

EXCEPTION IS 3 DEGREE ALKYL HALIDE CAN REACT VIA E2 in PRESENCE OF STRONG BASE (NAOH, KOH, CH3O)

weak base

2 alkyl halide will react via E1 with ____

strong base

2 alkyl halide will react via E2 with

3 (Fastest) ——> 1 (Slowest)

E1 speed

1 (Fastest) ——> 3 (Slowest)

E2

minor

BULKY BASE MEANS THE _____ PRODUCT IS FAVORED