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

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 (Parent must contain functional group!) 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 b) 3,4,4-Trimethyl-1-pentene Naming alkenes 3-Methyl-3-hexene

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) 2,5-Dimethyl-3-hexyne B) 3,3-Dimethyl-1-butyne

3.2 Electronic Structure of Alkenes Rotation of  bond is prohibited  bond must break for rotation to occur (unlike a carbon-carbon single bond). Creates possible alternative structures – cis and trans isomers

3.3 Cis-Trans Isomers of Alkenes Carbon atoms in a double bond are sp2-hybridized Three equivalent orbitals at 120º Fourth orbital is atomic p orbital Combination of electrons in two sp2 orbitals of two atoms forms  bond between them Overlap of p orbitals creates a  bond  bond prevents rotation about -bond High barrier to rotation, 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 Why?

Worked Example Which compound is more stable? a) Stability of alkenes

Worked Example Which compound is more stable? b) Stability of alkenes

Cis-Trans Isomers of Alkenes (Continued) Cis-Trans Isomer nomenclature 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 How do you distinguish between these two molecules for example? 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 Assign priority to two groups on each carbon and designate where two highest priority groups appear using 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 attached to each double bond carbon based on their 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, continue outward to next atom Compare until a difference in atomic number is detected 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 There are many different classes of reactions in organic chemistry Addition reactions – two molecules combine Elimination reactions – one molecule splits into two

Kinds of Organic Reactions (Continued) Substitution reactions– 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 an organic reaction, the transformation that occurs is evident in considering the reactant and product The mechanism of the reaction describes the steps that are involved in causing the changes that are observed Reactions occur in defined steps that lead from reactant to product – invariably this involves the flow of electrons from one atom to another (bonds reorganize in going from reactant to product and this involves the movement of electrons)

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‘radical processes’) – one electron processes (homolytic) Arrowheads with a “double head” indicate ‘polar processes’) – two electron processes (heterolytic)

Radical Reactions (Unpaired electrons) Radical reactions not as common as polar reactions 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 “phile” means loving

3.7 The Mechanism of an Organic Reaction: Addition of HBr to an Alkene 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 an Akene Double Bond HBr electrophile is attacked by  electrons of the alkene double bond (nucleophile) to form a carbocation intermediate and bromide ion Bromide adds to the positive center (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 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 provides a mechanism with a lower activation energy. Example: Hydrogenation of double bond with Pd