Chapter 3 Alkenes and Alkynes: The Nature of Organic Reactions Suggested Problems: 21,25,26-32,35,40-2,43,47,51,54,56,58

Alkene - Hydrocarbon With Carbon-Carbon Double Bond Hydrocarbon that contains a C=C double bond Sometimes called an olefin but alkene is better Includes many naturally occurring materials Flavors, fragrances, vitamins

Alkyne - Hydrocarbon With Carbon-Carbon Triple Bond Hydrocarbon that contains a C≡C triple bond Less common than alkenes Simplest member acetylene Alkenes & alkynes are said to be unsaturated

3.1 Naming Alkenes and Alkynes Name the parent hydrocarbon using suffix –ene in place of -ane Number the carbons in chain so that double bonded carbons have lowest possible numbers Note – the branch point gets the lowest number above right

Naming Alkenes (Continued) Write the full name- Number substituents according to: 1) Position in chain, 2) Alphabetically Rings have “cyclo” prefix

Many Alkenes Are Known by Common Names

Worked Example Give IUPAC names for the following compounds a) b) Naming alkenes

Worked Example Solution: a) The compound is 3,4,4-Trimethyl-1-pentene The longest chain is a pentene Consists of three methyl groups The methyl groups are attached to C3 and C4 Using tri- before methyl and using numbers to signify the location of the double bond The compound is 3,4,4-Trimethyl-1-pentene Naming alkenes

Worked Example b) The compound is 3-Methyl-3-hexene The longest chain is a hexane The methyl group is attached to C3 Using numbers to signify the location of the double bond The compound is 3-Methyl-3-hexene Naming alkenes

Naming Alkynes General hydrocarbon rules apply with “-yne” as a suffix indicating an alkyne Numbering of chain with triple bond is set so that the smallest number possible is assigned to the first carbon of the triple bond

Worked Example Name the following alkynes: A) B) Naming alkynes

Worked Example Solution: A) 2,5-Dimethyl-3-hexyne B) 3,3-Dimethyl-1-butyne Naming alkynes

3.2 Electronic Structure of Alkenes Rotation of  bond is prohibitive  bond must break for rotation to occur (unlike a carbon-carbon single bond). Creates possible alternative structures

3.3 Cis-Trans Isomers of Alkenes Carbon atoms in a double bond are sp2-hybridized Three equivalent orbitals at 120º separation in plane Fourth orbital is atomic p orbital Combination of electrons in two sp2 orbitals of two atoms forms  bond between them Additive interaction of p orbitals creates a  bonding orbital Occupied  orbital prevents rotation about -bond Rotation prevented by  bond - high barrier, about 268 kJ/mole in ethylene

Cis-Trans Isomers of Alkenes (Continued) 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 cis alkenes are less stable than trans alkenes

Worked Example Which compound in each pair is more stable a) b) Stability of alkenes

Worked Example Solution: a) b) Stability of alkenes

Cis-Trans Isomers of Alkenes (Continued) Cis-Trans Isomerization requires that end groups differ in pairs Bottom pair cannot be superposed without breaking C=C

3.4 Sequence Rules: The E,Z Designation Cis-Trans naming system discussed thus far only works with disubstituted alkenes Tri- and Tetra substituted double bonds require more general method Method referred to as the E,Z system

Sequence Rules: The E,Z Designation (Continued): E,Z Stereochemical Nomenclature Priority rules of Cahn, Ingold, and Prelog Compare where higher priority groups are with respect to bond and designate as prefix E -entgegen, opposite sides Z - zusammen, together on the same side Z zame zide

Sequence Rules: The E,Z Designation (Continued): Cahn-Ingold-Prelog Rules Rank substituent atoms according to atomic number of first atom Higher atomic number gets higher priority Br > Cl > S > P > O > N > C > H

Sequence Rules: The E,Z Designation (Continued): Cahn-Ingold-Prelog Rules 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

Sequence Rules: The E,Z Designation (Continued): Cahn-Ingold-Prelog Rules Multiple-bonded atoms are equivalent to the same number of single-bonded atoms Substituent is drawn with connections shown and no double or triple bonds

Worked Example Assign stereochemistry (E or Z) to the double bond in the following compound Convert the drawing into a skeletal structure (red = O) Alkene Stereochemistry and the E,Z Designation

Worked Example Solution: Alkene Stereochemistry and the E,Z Designation

3.5 Kinds of Organic Reactions Common patterns describe the changes in organic reactions Addition reactions – two molecules combine Elimination reactions – one molecule splits into two

Kinds of Organic Reactions (Continued) Substitution – parts from two molecules exchange

Kinds of Organic Reactions (Continued) Rearrangement reactions – a molecule undergoes changes in the way its atoms are connected

3.6 How Reactions Occur: Mechanisms In a clock the hands move but the mechanism behind the face is what causes the movement In an organic reaction, we see the transformation that has occurred. The mechanism describes the steps behind the changes that we observe Reactions occur in defined steps that lead from reactant to product – invariably this involves the flow of electrons from one atom to another

Steps in Mechanisms We classify the types of steps in a sequence A step involves either the formation or breaking of a covalent bond Steps can occur individually or in combination with other steps When several steps occur at the same time, they are said to be concerted

Types of Steps in Reaction Mechanisms Bond formation or breakage can be symmetrical or unsymmetrical Symmetrical- homolytic Unsymmetrical- heterolytic

Indicating Steps in Mechanisms Curved arrows indicate breaking and forming of bonds Arrowheads with a “half” head (“fish-hook”) indicate homolytic and homogenic steps (called ‘radical processes’) – one electron processes Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’) – two electron processes

Radical Reactions (Unpaired electrons) Not as common as polar reactions Radicals react to complete electron octet of valence shell A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule A radical can add to an alkene to give a new radical, causing an addition reaction

Polar Reactions (paired electrons) Molecules can contain local unsymmetrical electron distributions due to differences in electronegativities This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom The more electronegative atom has the greater electron density Elements such as O, F, N, Cl are more electronegative than carbon

Generalized Polar Reactions An electrophile, an electron-poor species, combines with a nucleophile, an electron-rich species An electrophile is a Lewis acid (+ or d+) A nucleophile is a Lewis base (electron pair) The combination is indicated with a curved arrow from nucleophile to electrophile

Some Nucleophiles and Electrophiles

3.7 The Mechanism of an Organic Reaction: Addition of HBr to Ethylene HBr (Note: The text uses HCl.) adds to the  part of a C-C double bond The  bond is electron-rich, allowing it to function as a nucleophile H-Br is electron deficient at the H since Br is much more electronegative, making HBr an electrophile

Mechanism of Addition of HBr to Ethylene HBr electrophile is attacked by  electrons of ethylene (nucleophile) to form a carbocation intermediate and bromide ion Bromide adds to the positive center of the carbocation, which is an electrophile, forming a C-Br  bond The result is that ethylene and HBr combine to form bromoethane All polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile

3.8 Describing a Reaction: Transition States and Intermediates The highest energy point in a reaction step is called the transition state The energy needed to go from reactant to transition state is the activation energy (Eact)

Describing a Reaction: Transition States and Intermediates If a reaction occurs in more than one step, it must involve species that are neither the reactant nor the final product These are called reaction intermediates or simply “intermediates” Each step has its own free energy of activation The complete diagram for the reaction shows the free energy changes associated with an intermediate

Hydrogenation of double bond with Pd Effect of a Catalyst A catalyst is a substance that increases the rate of a reaction by providing an alternative mechanism. The catalyst participates in the reaction. It is regenerated during the reaction. The catalyst provided a mechanism with a lower activation energy. Example: Hydrogenation of double bond with Pd