Structure of Alkenes

1
Structure of Alkenes

• Alkenes (and alkynes) are unsaturated
  hydrocarbons
• Alkenes have one or more double bonds
• The two bonds in a double bond are different
• - one bond is a sigma (?) bond these are
  cylindrical in shape and are very strong
• - the other is a pi (p) bond these involve
  sideways overlap of p-orbitals and are weaker
  than ? bonds
• Alkenes are flat and have a trigonal planar shape
  around each of the two Cs in a double bond

2
(No Transcript)
3
(No Transcript)
4
Alkenes

• Structure
• the VSEPR model predicts bond angles of 120
  about each carbon of a double bond
• in ethylene, the actual angles are close to 120
• in substituted alkenes, angles about each carbon
  of the double bond may be greater than
  120 because of repulsion of alkyl groups bonded
  to the double bond

5
Structure of Alkynes

• Alkynes have one or more triple bonds
• A triple bond consists of one ? bond and two p
  bonds
• - the two p bonds are orthogonal (perpendicular)
• Alkynes are linear around each of the two Cs in
  the triple bond
• Because alkenes and alkynes have p bonds, which
  are much weaker than ? bonds, they are far more
  chemically reactive than alkanes

6
(No Transcript)
7
Naming Alkenes and Alkynes

• Parent name ends in -ene or -yne
• Find longest chain containing double or triple
  bond
• Number Cs starting at end nearest multiple bond
• Locate and number substituents and give full name
• - use a number to indicate position of multiple
  bond
• - cycloalkenes have cyclo- before the parent
  name numbering begins at double bond, giving
  substituents lowest possible numbers
• - use a prefix (di-, tri-) to indicate multiple
  double bonds in a compound

8
Alkenes - IUPAC Names
9
Cycloalkenes

• To name a cycloalkene
• number the carbon atoms of the ring double bond 1
  and 2 in the direction that gives the lower
  number to the substituent encountered first
• number and list substituents in alphabetical
  order

10
Dienes, Trienes, Polyenes

• alkenes that contain more than one double bond
  are named as alkadienes, alkatrienes, and so on
• those that contain several double bonds are
  referred to more generally as polyenes (Greek
  poly, many)

11
Alkynes - IUPAC Names
12
(No Transcript)
13
Common Names

• Common names are still used for some alkenes and
  alkynes, particularly those of low molecular
  weight

14
Cis-Trans Isomers of Alkenes

• The p bond gives an alkene a rigid structure
• Free rotation around the C-C bond is not possible
  because the p bond would have to break and
  re-form
• So, groups attached to the double bond are fixed
  on one side or the other
• If each C in the double bond has two different
  groups attached, then cis-trans isomers are
  possible
• - Cis 2 groups attached to the same side of
  the double bond
• - Trans 2 groups attached to opposite sides of
  the double bond

15
(No Transcript)
16
Physical Properties

• alkenes and alkynes are nonpolar compounds
• the only attractive forces between their
  molecules are London dispersion forces
• their physical properties are similar to those of
  alkanes with the same carbon skeletons
• alkenes and alkynes are insoluble in water but
  soluble in one another and in nonpolar organic
  liquids
• alkenes and alkynes that are liquid or solid at
  room temperature have densities less than 1 g/mL
  they float on water

17
Addition Reactions of Alkenes and Alkynes

• Addition (combination) reactions have the form
• A B ? AB
• For alkenes the general reaction has the form
• R2CCR2 A-B ? R2AC-CBR2
• (where R any alkyl group or H)
• Addition reactions are the most common types of
  reactions for alkenes and alkynes
• The p bonds are easily broken, and that pair of
  electrons can form a new ? bond
• The reactions are favorable because the products
  (all ? bonds) are more stable than the reactants

18
(No Transcript)
19
Hydrogenation of Alkenes and Alkynes

• H2 can be added to alkenes or alkynes to form
  alkanes
• Usually a metal catalyst (Pt, Pd or Ni) is used
  to speed up the reaction (the reaction generally
  doesnt work without a catalyst)
• Because these reactions take place on a surface,
  hydrogenation of substituted cycloalkenes
  produces cis products.

20
Addition of H2 - Reduction

• Virtually all alkenes add H2 in the presence of a
  transition metal catalyst, commonly Pd, Pt, or Ni

21
Hydrohalogenation of Alkenes

• Hydrogen halides (HCl, HBr or HI) can add to
  alkenes to form haloalkanes
• When a hydrogen halide adds to a substituted
  alkene, the halide goes to the more substituted C
  (Markovnikovs rule)

22
Addition of HX

• reaction is regioselective
• Markovnikovs rule H adds to the less
  substituted carbon and X to the more substituted
  carbon

23
Mechanism of hydrohalogenation

• Hydrohalogenation takes place in two steps
• In the first step, H is transferred from HBr to
  the alkene to form a carbocation and bromide ion
• Second, Br- reacts with the carbocation to form a
  bromoalkane
• Example

24
(No Transcript)
25
Addition of Water to Alkenes

• In the presence of a strong acid catalyst (HCl,
  H2SO4 etc.) alkenes react with H2O to form
  alcohols
• Recall that acids form H3O in water it is the
  H3O that reacts with the alkene
• Hydration reactions follow Markovnikovs rule

26
Mechanism of Acid-Catalyzed Alkene Hydration

• First, the alkene reacts with H3O to form a
  carbocation
• Next an H2O quickly reacts with the carbocation
  to form a protonated alcohol
• In the last step the proton is removed by an H2O
  to form an alcohol

27
Halogenation of Alkenes and Alkynes

• Halogens (Cl2 or Br2) can add to alkenes or
  alkynes to form haloalkanes
• Alkenes form dihaloalkanes alkynes form
  tetrahaloalkanes
• Reaction with cycloalkenes produces a trans
  product

28
Addition of Cl2 and Br2

• Addition takes place readily at room temperature
• reaction is generally carried out using pure
  reagents, or mixing them in a nonreactive organic
  solvent
• addition of Br2 is a useful qualitative test for
  the presence of a carbon-carbon double bond
• Br2 has a deep red color dibromoalkanes are
  colorless

29
Mechanism of Bromonation of Ethene

• First, a Br is transferred from Br2 to the
  alkene to form a bromonium ion and a bromide ion
• Next, the bromide ion reacts with the bromonium
  ion to form the product

30
Polymers

• A polymer is a long chain of repeating subunits
  called monomers
• - examples of natural polymers DNA, protein,
  starch
• - example of synthetic polymers polyethylene
• Many synthetic polymers are made from alkenes,
  although other functional groups are also used
• The monomers are added to the chain through a
  series of addition reactions
• Polymerization reactions usually require high
  temperature and pressure and are often radical
  reactions carried out with a catalyst

31
Polymerization

• From the perspective of the organic chemical
  industry, the single most important reaction of
  alkenes is polymerization
• polymer Greek poly, many and meros, part
• monomer Greek mono, single and meros, part

32
Polymerization

• show the structure of a polymer by placing
  parentheses around the repeating monomer unit
• place a subscript, n, outside the parentheses to
  indicate that this unit repeats n times
• the structure of a polymer chain can be
  reproduced by repeating the enclosed structure in
  both directions
• following a section of polypropene
  (polypropylene)

33
(No Transcript)
34
(No Transcript)
35
(No Transcript)
36
Polyethylene

• Low-density polyethylene (LDPE)
• a highly branched polymer polymer chains do not
  pack well and London dispersion forces between
  them are weak
• softens and melts above 115C
• approximately 65 used for the production of
  films for packaging and for trash bags
• High-density polyethylene (HDPE)
• only minimal chain branching chains pack well
  and London dispersion forces between then are
  strong
• has higher melting point than LDPE and is
  stronger
• can be blow molded to squeezable jugs and bottles

37
Aromatic Compounds

• Aromatic compound a hydrocarbon that contains
  one or more benzene-like rings
• arene a term used to describe aromatic compounds
• Ar- a symbol for an aromatic group derived by
  removing an -H from an arene
• Kekulé structure for benzene (1872)

38
Conjugated Alkenes and Aromatic Compounds

• Recall that a double bond consists of one ? bond
  and one ? bond a ? bond is formed by sideways
  overlap of two p orbitals (one electron comes
  from each orbital)
• A conjugated alkene has alternating double and
  single bonds
• The p orbitals overlap in a conjugated system
  (the ? electrons are delocalized throughout the
  system), making conjugated alkenes more stable
  than non-conjugated alkenes
• An aromatic hydrocarbon consists of alternating
  double and single bonds in a flat ring system
• Benzene (C6H6) is the most common aromatic
  hydrocarbon
• In benzene all the double bonds are conjugated,
  and so the ? electrons can circulate around the
  ring, making benzene more stable than
  1,3,5-hexatriene (the p orbitals on the end of a
  chain can not overlap)

39
(No Transcript)
40
Resonance Structures

• There are two ways to write the structure of
  benzene
• These are called resonance structures
• However, neither of these represents the true
  structure of benzene since benzene has only one
  structure, with all C-C bonds being equivalent
• The true structure is a hybrid of the the two
  resonance structures this can be represented by
  drawing the ? bonds as a circle
• We use the individual resonance structures when
  we write reaction mechanisms involving benzene to
  show more clearly the bond formation and bond
  breaking in the reaction

41
Benzene

• Delocalized electrons are not confined between
  two adjacent bonding atoms, but actually extend
  over three or more atoms.

42
Naming Monosubstituted Benzene Compounds

• Benzene compounds with a single substituent are
  named by writing the substituent name followed by
  benzene
• Many of these compounds also have common names
  that are accepted by IUPAC (you should know those
  listed here)

43
Naming Multisubstituted Benzene Compounds

• When there are 2 or more substituents, they are
  numbered to give the lowest numbers (alphabetical
  if same both ways)
• Disubsituted benzenes are also named by the
  common prefixes ortho, meta and para

44
Nomenclature

• For three or more substituents
• if one of the substituents imparts a special
  name, name the molecule as a derivative of that
  parent
• if none of the substituents imparts a special
  name, number the substituents to give the
  smallest set of numbers, and list them in
  alphabetical order before the ending "benzene"

45
Nomenclature

• phenyl group (C6H5- or Ph-) the substituent
  group derived by loss of an H from benzene

46
PAHs

• Polynuclear aromatic hydrocarbon (PAH)
• a hydrocarbon that contain two or more benzene
  rings, with each pair of rings sharing two
  adjacent carbon atoms

47
Physical Properties of Aromatic Compounds

• Because aromatic compounds (like benzene) are
  flat, they stack well, and so have higher melting
  and boiling points than corresponding alkanes and
  alkenes (similar to cycloalkanes)
• Substituted aromatic compounds can have higher or
  lower melting and boiling points than benzene
• - para-xylene has a higher m.p. than benzene
• - ortho and meta-xylene have lower m.p.s than
  benzene
• Aromatic compounds are more dense than other
  hydrocarbons, but less dense than water
  (halogenated aromatics can be more dense than
  water, as can haloalkanes)
• Aromatic compounds are insoluble in water, and
  are commonly used as solvents for organic
  reactions
• Aromatic compounds are also flammable, and many
  are carcinogenic

48
Chemical Reactivity of Aromatic Compounds

• Aromatic compounds do not undergo addition
  reactions because they would lose their special
  stability (aromaticity)
• Instead, they undergo substitution reactions,
  which allow them to retain their aromaticity
• We will study three types of substitution
  reactions of benzene halogenation, nitration
  and sulfonation

49
Halogenation of Benzene and Toluene

• Br2 or Cl2 can react with benzene, using a
  catalyst, to form bromobenzene or chlorobenzene
• Only the monohalogenation product is produced
• When Br2 or Cl2 reacts with toluene, a mixture of
  isomers is produced
• - Ortho and para isomers are the major products,
  and meta isomer is the minor product

50
Mechanism of Bromonation of Benzene

• First, a Br is transferred from Br2 to benzene,
  forming a carbocation and a chloride ion
• Next, the chloride ion removes an H from the
  carbocation to form chlorobenzene and HBr

51
Nitration and Sulfonation of Benzene

• Nitric acid can react with benzene, using
  sulfuric acid as a catalyst, to form nitrobenzene
  plus water
• First H2SO4 donates a proton to HNO3, which then
  decomposes to form H2O and NO2 (the reactive
  species)
• Sulfur trioxide plus sulfuric acid (fuming
  sulfuric acid) can react with benzene to produce
  benzenesulfonic acid
• First H2SO4 donates a proton to SO3 to produce
  HSO3 (the reactive species)