Understanding Alkenes and Alkynes: Structure, Nomenclature, and Physical Properties - Prof, Study notes of Organic Chemistry

Download Understanding Alkenes and Alkynes: Structure, Nomenclature, and Physical Properties - Prof and more Study notes Organic Chemistry in PDF only on Docsity! 4-1© 2005 John Wiley & Sons, IncAll rights reserved Chapter 4: Alkenes and Alkynes 4-2© 2005 John Wiley & Sons, IncAll rights reserved hydrocarbons aliphatic aromatic saturated (only single C-C bonds) unsaturated (C-C multiple bonds) alkenes (C=C double bonds) alkynes (C≡C triple bonds) cycloalkanes (alicyclic, i.e., non-aromatic rings; C-C single bonds) alkanes (C-C single bonds) 4-5© 2005 John Wiley & Sons, IncAll rights reserved Nomenclature of Alkenes • In cycloalkenes, numbering starts with a carbon of the double bond • If multiple substituents are present, the carbon of the double bond closest to the first substituent is C1 • Some alkenes are known almost exclusively by their common names CH2=CH2 CH3CH=CH2 CH3C=CH2 CH3 IUPAC: IsobutylenePropyleneEthyleneCommon: 2-MethylpropenePropeneEthene 4-6© 2005 John Wiley & Sons, IncAll rights reserved Structure of Alkenes • The two carbon atoms of a double bond and the four atoms bonded to them lie in a plane, with bond angles of approximately 120° • VSEPR model H C C H H H 121.7° Ethylene H C C CH3 H H 124.7° Propene 4-7© 2005 John Wiley & Sons, IncAll rights reserved Physical Properties • Alkenes and alkynes are nonpolar compounds • the only attractive forces between their molecules are dispersion forces • Their physical properties are similar to those of alkanes of similar carbon skeletons • those that are liquid at room temperature are less dense than water (1.0 g/m L) • they dissolve in each other and in nonpolar organic solvents • they are insoluble in water 4-10© 2005 John Wiley & Sons, IncAll rights reserved sp2 hybridization sp2 hybridized carbon atom - three sp2 hybrid orbitals - 120 angle between orbitals - planar - one p orbital - 90 angle between orbitals p- and sp2 orbitals 4-11© 2005 John Wiley & Sons, IncAll rights reserved Structure of Alkenes • According to the orbital overlap model, the C=C double bond consists of • a -bond formed by overlap of sp2 hybrid orbitals • a -bond formed by overlap of parallel 2p orbitals all sigma bonds in ethene carbon-carbon double bond in ethene 4-12© 2005 John Wiley & Sons, IncAll rights reserved Structure of Alkenes all bonds in ethene (C2H4) each carbon makes four bonds • three sigma bonds • two C-H bonds (sp2-1s) • one C-C (sp2-sp2) • one pi bonds • one C-C (p-p) 4-15© 2005 John Wiley & Sons, IncAll rights reserved Structure of Alkenes • Rotation around sigma bonds requires little energy • <20 kJ/mol • Free rotation at room temperature • Rotating around pi bonds requires the breakage of the pi bond • 230-270 kJ/mol • Cis-trans isomerism 90 rotation around C-C bond 4-16© 2005 John Wiley & Sons, IncAll rights reserved Cis-Trans Isomerism • restricted rotation about a C=C double bond • groups on adjacent carbons are either • On the same side of the pi-bond => cis-isomer • On different sides of the pi-bond => trans-isomer • trans alkenes are more stable than cis alkenes • steric strain between alkyl substituents of the same side of the double bond in the cis configuration cis-2-Butene mp -139°C, bp 4°C t rans-2-Butene mp -106°C, bp 1°C C H3 C C H CH3 C HH C CH3 HH3 C Note: cis and trans 2-Butene are stereoisomers (not constitutional isomers!) 4-17© 2005 John Wiley & Sons, IncAll rights reserved Configuration: E,Z • The E,Z system: used to name tri- and tetrasubstituted alkenes • A priority is assigned to each substituent on the carbon atoms participating in the double bond • If the two substituents with higher priority are on the same side of the double bond => the configuration is Z (German: zusammen, together) • If the two substituents with higher priority are on opposite sides of the double bond => the configuration is E (German: entgegen, opposite) Z (zusammen) E (entgegen) C higher C higher lowerlower C lower higher C lowerhigher 4-20© 2005 John Wiley & Sons, IncAll rights reserved Configuration - E,Z 3. Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds -CH=CH2 O -CH O H C C O CC -CH-CH2 is treated as is treated as 2 1 1 2 C=C HC=CH2 C2H5HO-CH2 O=CH 4-21© 2005 John Wiley & Sons, IncAll rights reserved Physical Properties of E/Z Isomers • Consider 1,2-Dichloroethene polar => higher boiling point worse packing => higher melting point nonpolar => lower boiling point better packing => higher melting point 4-22© 2005 John Wiley & Sons, IncAll rights reserved Stability of Alkenes • Electronic effect (larger) • The larger the number of alkyl groups on a C-C double bond, the more stable the molecule • 2-Butene is more stable than 1-Butene • Steric effect (smaller) • Repulsion between electrons reduces stability (Van-der-Waals strain) • Trans alkenes are more stable than cis alkenes 2 alkyl groups 1 alkyl group 4-25© 2005 John Wiley & Sons, IncAll rights reserved Dienes, Trienes, polyenes • for an alkene with n carbon-carbon double bonds, =>up to 2n cis-trans isomers • alkenes with 2 double bonds are called dienes • alkenes with 3 double bonds are called trienes • consider 2,4-heptadiene; it has four cis-trans isomers, two of which are drawn here • Nomenclature: same rules apply as for alkenes with one double bond; use numbers to indicate location of the double bonds C2 -C3 C4 -C5 Double bond trans trans trans cis cis trans cis cis t rans,t rans-2,4- heptadiene trans ,cis-2,4- heptadiene 2 2 4 4 4-26© 2005 John Wiley & Sons, IncAll rights reserved Vitamin A • vitamin A has five double bonds • can show cis-trans isomerism • vitamin A is the all-trans isomer Vitamin A aldehyde (retinal) enzyme- catalyzed oxidation H O Vitamin A (retinol) OH Rhodopsin (visual purple): - retinal bonded to protein opsin - light sensitive pigment in rods and cones of the retina 4-27© 2005 John Wiley & Sons, IncAll rights reserved Vitamin A • Absorption of light changes the cis-configuration on C- 11 to the trans-configuration • An enzyme catalyzes the transformation of the all-trans retinal to the 11-cis retinals WHY? HOW? 4-30© 2005 John Wiley & Sons, IncAll rights reserved Structure of Alkynes • The functional group of an alkyne is a carbon- carbon triple bond • A triple bond consists of • one  bond formed by the overlap of sp hybrid orbitals • two  bonds formed by the overlap of sets of parallel 2p orbitals 4-31© 2005 John Wiley & Sons, IncAll rights reserved sp Bonding in Alkynes • Each carbon atom of the triple bond has two sp and two p-orbital • sp orbitals: one -bond to carbon and one to hydrogen • p-orbitals: two -bonds to carbon of triple bond en er g y 1s 2s 2px 2py 2pz ground state 1s 2s 2px 2py 2pz promotion of one 2s electron 1s sp sp- hybridization two sp-hydrid orbitals for -bonds p p p-orbitals for  bonds 4-32© 2005 John Wiley & Sons, IncAll rights reserved sp hybridization +  two sp hybrid orbitals two p-orbitals • the two sp-orbitals make C-C or C-H -bonds • the two p-orbitals make the two -bonds of the C≡C triple bond • each sp orbital has 50% p-character and 50% s-character 4-35© 2005 John Wiley & Sons, IncAll rights reserved Nomenclature of Alkynes • IUPAC nomenclature • use the suffix -yne to show the presence of a carbon- carbon triple bond • For C2H2, there are two acceptable IUPAC names: • acetylene and ethyne • number the parent chain to give the 1st carbon of the triple bond the lower number • follow IUPAC rules for numbering and naming substituents 3-Methyl-1-butyne 6,6-Dimethyl-3-heptyne 1 1 2 2 3 3 4 4 5 6 7 4-36© 2005 John Wiley & Sons, IncAll rights reserved Nomenclature of Alkynes • If more than one triple bond present, use –diyne, - triyne, tetrayne etc. • If double and triple bonds are present • Use –enyne as suffix • Number from side nearest a multiple bond 4-37© 2005 John Wiley & Sons, IncAll rights reserved Summary: Alkanes, Alkenes, Alkynes =268 kJ/mol =201 kJ/mol