alkenes hand out 1

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SAS SGUFALL 2012CHEM 222 ALKENES: NOMENCLATURE,STRUCTURE, ISOMERISM

{WADE: Chapter 7}

A Carbon-Carbon double bond is an important functionalgroup. The double bond is the site of most of the chemicalreactions that alkenes undergo. Alkenes play many importantroles in Biology; e.g. ethene is a plant hormone.

The double bond in ethene is stronger than the C C single bond in ethane, but it not twiceas strong; the bond is stronger than the bond (sigma bond energy = 347 kJ/mol; pibond energy = 264 kj/mol).

The general molecular formula for an acyclic (noncyclic) alkene is CnH2n; while for a cyclicalkene it is CnH2n-2.Thetotal number of bonds and/or rings in an alkene is called the element(degree) ofunsaturation. So,the acyclic alkene C5H10 has one (1) degree of unsaturation; while the cyclicalkene C5H8 has two degrees of unsaturation. Alkenes are unsaturated hydrocarbons.

Formula for calculating element of unsaturation is: (2C+2 H), where C= number ofcarbons, H= number of hydrogens

{(a)Work out the element of unsaturation for the following formulas: (i) C10H16 (ii)C20H34 (iii) C40H56.(b) Determine the molecular formula for (i) a 5-carbon hydrocarbon with one bondand one ring; (ii) a 10-carbon hydrocarbon with one bond and two rings}

STRUCTURE of ALKENES

Recall the structure ofethene (H2C=CH2) from the hybridization lecture/handout. The bondconsists of the cloud of electrons distributed above and below the plane defined by the two sp2carbons and the four atoms bonded to them.In ethene, C2H4, three atoms are attached to each carbon atom, so three equivalent hybridorbitals are needed. The 2s orbital is mixed with two of the 2p orbitals in sp2 hybridization. (One

of the 2p orbitals with its electron is leftunhybridized). Each sp2 orbital has 1/3 scharacter and 2/3 p character. The axes ofthe sp2 orbitalsare co-planar

giving anangle of 1200.For each carbon atom in ethene: two of the sp2orbitals bond with

two H atoms in sigma () bonds; the 3rd overlaps with the third sp2

orbital on the second carbon atom,also in a sigma () bond. Whatsleft is one unhybridized 2p orbitalon each carbon atom. What dothey do? They overlap forming a

pi ( ) bond. So the double bond


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between the two C atoms in ethene contains one sigma () bond and one pi ( ) bond. In orderto achieve maximum orbital-orbital overlap, the twop orbitals must be parallel to each other.

Therefore all six atoms of the double-bond system are in the same plane.

NOMENCLATURE of ALKENES

All the IUPAC rules you learned for alkanes and alkyl halides apply to alkenes as well.1.The longest continuous chain containing the functional group, the C = C, is numbered in the

direction to give it (the functional group suffix) the lowest possible number{alternative way to namealkenes: but-1-ene, hex-2-ene, etc.}

2. For a compound with two double bonds, the suffix is diene; CH2=CHCH=CH2 is 1,3-butadiene.

3. When there is both a functional group and a substituent, the functional group(the C=C) gets

the lowest possible number

4. If a chain has more than one substituent, the alphabet rule applies CH3CH2CH(Br)CH(Cl)CH=CHCH3is 5-bromo-4-chloro-2-heptene

5. If counting in either direction results in the same number for the C=C group, the correct name

is the one containing the lowest substituent number

6. In a cyclic alkene, the C=C is always betweenC1 and C2, so the 1 is not required. Toassign numbers to any substituents, count around the ring in the direction (clockwise or anti-

clockwise) that putsthe lowest number intothe name.


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The sp2 carbons of an alkene are called vinylic carbons. A sp3 carbon that is adjacent to avinylic carbon is called an allylic carbon. Two groups containing a C=C are used in common

names- the vinyl group (CH2=CH-) and the allyl group(CH2=CHCH2-)

The =CH2substituentis the methylene group, so the =CH2attached to a cyclohexane ring ismethylene cyclohexane.

Cis- & Trans- ISOMERISMRotation about a double bond does notoccur. Because of the high energy barrierto rotation about a C=C bond, an alkenesuch as 2-butene can exist in twodifferent forms: the hydrogens bondedto the sp2 carbons can be on the sameside of the double bond or on oppositesides of the double bond.

The compound with the Hs on the same

side of the C=C is the cis-isomer, and thecompound with the Hs on opposite sidesof the C=C is the trans-isomer. A pair of isomers such as cis-2-butene and trans-2-buteneare called geometric (or cis-trans) isomers. Notice that the cis- and trans- isomers have the

same bonds but different configurationsthey differ in the way the atoms are oriented inspace.

The two isomers are different compoundswith different physical properties (boiling points,

dipole moments)


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Note that if either carbon of the double bond holds two identical groups (or atoms),the molecule cannot have cis and trans forms (see below right)

The E, ZSYSTEM of NOMENCLATURE for ISOMERS

The E, Zsystem is used to name alkenes with two (or more) substituents on one orboth of the sp2 carbons. To name an isomer in the E, Zsystem, we first determine therelative priorities of the two groups bonded to each of the sp2 carbons. If the two high-priority groups (one from each carbon) are on the same side of the double bond, it is theZisomer (Zis for zusammen, German for together). If the high-priority groups are on

opposite side of the double bond, itis the E isomer (Eis forentgegen, German foropposite).

The relative priorities of the two groups bonded to a sp2 carbon are determined using therules below:

1. The relative priorities of the two groups depend onthe atomic numbers of the atoms bonded directly tothe sp2carbons. The greater the atomic number, thehigher the priority.

2. If the two groups bonded to a sp2 carbon startwiththe same atom (there isa tie), move outwardfromthe point of attachment and consider the atomic

numbers of the atoms attached to the tiedatoms. The one with the greater atomic numberbelongs to the group with the higher priority.

3(a). If an atom is doubly bonded to anotheratom, the priority system treats it as if it weresingly bonded to two of those atoms. If anatom is triply bonded to another atom, thepriority system treats it as if it were singlybonded to three of those atoms.


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3(b). Also, when two groups competing for the higher priority have some identicalatoms, those atoms cancel each other out, and the atomic numbers of the remainingatoms are used to determine which group has higher priority.

4. Iftwo isotopes are being compared, the isotope with the greater mass number has thehigher priority

PHYSICAL PROPERTIES of ALKENES

Alkenes are similar to alkanes in most of their physical properties.(i) The lower molecular mass alkenes through C4H8 are gases @ room temperature &atmospheric pressure.(ii) The boiling points of 1-butene, cis-2_butene, trans-2-butene and n-butane are all close to 00C.As with alkanes, boiling points of alkenes increase smoothly with increasing molecular mass.Also, increased branching leads to greater volatility and lower boiling points; e.g. 2-methylpropene has a boiling point of -70C, lower than the boiling point of any of the unbranchedalkenes.(iii) Alkenes have densities around 0.6 or 0.7 g/cm3.

(iv)Like alkanes, alkenes are relatively nonpolar but they are more polar than alkanes. They areinsoluble in water but soluble in organic solvents. Alkenes are more polar than alkanes because:the more weakly held electrons in the bond are more polarizable, and vinylic bonds tend to beslightly polar (contributing to a permanent dipole moment).

The dipole moments of most alkenes are quite small. Among the C4H8 isomers: 1-butene,cis-2-butene, and 2-methylpropene have dipole moments in the 0.3-0.5 D range; trans-2-butenehas no dipole moment.

Alkyl groups release electrons to a C=C in much the same way that they releaseelectrons to the positively charged carbon of a carbocation.

Therefore the more alkyl groups attached to the carbon atoms in the double bond, themore stable is the alkene. Among the C4H8 isomers, the data indicates that stabilityincreases in the order

1-butene < cis-2-butene < trans-2-butene < 2-methylpropene.

Therefore, the more highly substituted the carbon atoms in the carbon carbondouble bond are the more stable the alkene is.

Richard JacquesOctober 2012

alkenes hand out 1

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