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Topic 5B Bonding in carbon compounds 3 sp 9 hybridization • This is the reason why carbon is tetrahedral in many compounds • By hybridization of its valence atomic orbitals, carbon can bond in a variety of ways First look at the normal electronic configuration of carbon: ml = -1 E 0 1 px py pz l=1 2p l=0 n=2 2s Valence shell 2s2 2p2 s 3 sp hybridization • Promote one 2s electron into the vacant p-orbital. • Combine (mix) all four orbitals to give four hybrid orbitals of equivalent energy: EE 2p 2p 2sp3 2s 2s 9 3 sp hybridization 9 • Each sp3 hybrid orbital has 25% “s” and 75% “p” character • Each sp3 hybrid orbital looks like a distorted dumbell: + 2s 2p sp3 hybrid sp3 Hybridization Animation 9 QuickTime™ and a Graphics decompressor are needed to see this picture. Movie from Saunders General Chemistry CDROM The best arrangement of orbitals is a tetrahedral geometry making angles of 109° 10 Tetrahedral bonding • Each sp3 hybrid orbital has one electron and can form a strong covalent bond with another atom, eg methane formation with four hydrogens: H H H C C H C H H H H H methane H H 109.5° H Sigma () bonds • The H 1s and carbon sp3 hybrid orbitals are no longer separate entities and combine to form a sigma () bonding molecular orbital. • These bonds are 109.5° apart. H C H 109.5° H H 10 Sigma () bond formation QuickTime™ and a Video decompressor are needed to see this picture. Movie from Saunders General Chemistry CDROM 10 Other representations Ball and stick QuickTime™ and a Animation decompressor are needed to see this picture. Space Filling QuickTime™ and a Animation decompressor are needed to see this picture. 10 Other representations Space Filling Potential Energy Surface 10 C–C bond formation in Ethane H H H C C H H 11 H Ethane Sigma () bonds can be formed between two carbons by overlapping two sp3 hybrid orbitals. H H C H H H + C H H H H C H C H H sp3 - sp3 bond between carbons Ethane Ball and stick QuickTime™ and a Animation decompressor are needed to see thi s picture. 11 Space filling QuickTime™ and a Animation decompressor are needed to see this picture. Ethane Ethane can spin about the C—C bond There is nearly free rotation: QuickTime™ and a Animation decompressor are needed to see this picture. 11 Propane 11 Propane is formed by covalent bonding to two other carbons and eight hydrogens. Ball and stick Space filling QuickTime™ and a Animation decompressor are needed to see thi s picture. QuickTime™ and a Animation decompressor are needed to see this picture. Propane Propane can rotate about both C—C bonds QuickTime™ and a Animation decompressor are needed to see this picture. QuickTime™ and a Animation decompressor are needed to see this picture. 11 Butane 11 7 Butane is formed by covalent bonding between four carbons and ten hydrogens. QuickTime™ and a Animation decompressor are needed to see thi s picture. Ball and stick QuickTime™ and a Animation decompressor are needed to see this picture. Space filling Butane 11 7 Butane can rotate about all three C—C bonds QuickTime™ and a Animation decompressor are needed to see this picture. QuickTime™ and a Animation decompressor are needed to see this picture. QuickTime™ and a Animation decompressor are needed to see this picture. Bonding to other atoms 12 • Alcohols are formed between sp3 hybridised carbon and oxygen: H H C H H H + O H H C O H sp 3 - sp 3 valence bond between carbon and oxygen giving an alcohol Bonding to other atoms 12 • Amines are formed between sp3 hybridised carbon and nitrogen: H H C H H H + N H H H C N H H sp 3 - sp 3 valence bond between carbon and nitrogen giving an amine sp2 Hybridization Double bond formation 13 • Carbon can form double bonds with itself and other heteroatoms. • This requires sp2 hybridization of its valence atomic orbitals. • Carbon is sp2 hybridized in: H H C H H C H Ethene (carbon sp2) C O H Formaldehyde (carbon, oxygen sp2) 2 sp 13 Hybridization • Promote one 2s electron into the vacant p-orbital. • Combine (mix) the 2s, 2px and 2py orbitals to give three hybrid orbitals of equivalent energy • The 2pz orbital is unaltered. EE x 2p y z 2p z 2p 2s 2s combine 2sp 2 2 sp Hybridization 13 • Only the 2px and 2py combine with the 2s orbital. • The three hybrid orbitals make angles of 120° to minimise electron repulsion between them. 120° 2s 2p y 2p x 120° 120° 3 sp 2 hybrid orbitals Trigonal planar carbon 13 • There are four electrons — one in each orbital • Note that the 2pz orbital is unchanged and perpendicular to the plane of the hybrid system. 2 sp hybrid C2pz 120° sp2 hybrid 120° 2 sp hybrid 120° An sp2 hybridised carbon atom. Pi () bonding Ethylene • Two sp2 carbons can form a covalentbond. • Other hybrid orbitals covalently bond to four hydrogens. C2pz C2pz 13 Pi () bonding Ethene 14 • Less efficient sideways overlap of the pz orbitals gives a second C—C bond — a pi () bond. • Both clouds (shown in green and blue) are part of the same -bonding orbital. C2pz C2pz H CH bonds H H H CC bond CH bonds H H H H CC bond Pi () bonding Ethene animation QuickTime™ and a Graphics decompressor are needed to see this picture. Movies from Saunders General Chemistry CDROM 14 Pi () bonding (theoretical approach) • Overlap of two C2pz atomic orbitals forms two pi molecular orbitals, (lower in energy) and * (higher in energy). • The electrons in C2pz orbitals are stabilised by occupying the lower energyorbital. p H 14 H One *-molecular H orbital H E * C2pz C2pz H H H H One -molecular orbital Ethylene Because each carbon is trigonal planar, ethylene is a flat molecule with thickness due to the pi-electrons. QuickTime™ and a Animation decompressor are needed to see thi s picture. 14 Ethylene The pi-bond restricts rotation about the C=C bond. A little twisting is possible but it is essentially rigid. QuickTime™ and a Animation decompressor are needed to see thi s picture. 14 15 Ethylene Geometry of ethylene: 121Þ H C H C 118Þ H 134 pm H CH3 CH3 154 pm Other double bonded systems: H H C O H Formaldehyde C N H H An imine sp Hybridization Alkyne formation 15 • Carbon can form triple bonds with itself and with other heteroatoms (eg in H—C. • This requires sp hybridization of its valence atomic orbitals. • Carbon is sp hybridized in ethyne, also called acetylene: H C C Ethyne (carbon sp) H 15 sp Hybridization • Promote one 2s electron into the vacant p-orbital. • Combine (mix) the 2s and 2px orbitals to give two hybrid orbitals of equivalent energy • The 2py and 2pz orbital are unaltered. E E x 2p y z 2py 2p 2s 2s combine 2sp 2pz sp Hybridization • Only the 2px combines with the 2s orbital. • The two hybrid orbitals make angles of 180° to minimise electron repulsion between them. 180° 2s 2px Two colinear sp hybrid orbitals 15 sp hybridised carbon • The two hybrid orbitals are semi-occupied • Note that the 2pz and 2py orbitals are unchanged and perpendicular to the plane of the hybrid system. C2pz An sp hybridised carbon atom sp hybrid C2py sp hybrid 15 Triple bonding in Ethyne • Two sp hybridised carbons can form a covalentbond. • Other hybrid orbitals covalently bond to two hydrogens. C2pz C2py C2pz C2py 16 Pi () bonding in Ethyne 16 • Less efficient sideways overlap of the pz and py orbitals gives two C—C pi () bonds . • These together with the bond form the triple bond. • Two sets of clouds (shown in green and blue) form y and z bonding orbital. y C2pz C2pz CH bond CH bond CC bond C2py C2py H C C H z Ethyne (acetylene) • Because each carbon is sp hybridised (hybrid orbitals 180° apart) , ethyne is a linear molecule. • Pi bonds form a barrel of electron density around the CC bond. QuickTime™ and a Animation decompressor are needed to see thi s picture. 16 Bond length—strength CC bonds Summary: • Bond length decreases from single to double to triple bond. • Bond strength increases from single to double to triple bond. pm 17 Functional Groups Alcohols 18 H H H H C H O H3C O CH3OH Methanol Functional Groups Alcohols R .. OH .. CH3CH2OH Ethanol R = alkyl group, OH = hydroxyl group 18 Functional Groups Alcohols Classification: One R group R CH2 OH R CH OH Primary ( 1 °) Secondary ( 2 °) Two R groups R R Three R groups R C R OH Tert iary ( 3 °) 18 Functional Groups Amines 19 • In methylamine, sp3 nitrogen is covalently bonded to methyl and two hydrogens N H H C N H CH3 H H Methylamine (Methanamine) H H Functional Groups Amines • Classified on number of alkyl groups attached to nitrogen R NH2 1 hydrogen replaced Primary (1°) amine 2 hydrogens replaced Secondary (2°) amine 3 hydrogens replaced Tertiary(3°) amine H N R R' R'' N R R' 19 Functional Groups Ketones and Aldehydes R CHO R = organic group, CHO = aldehyde group R R CO R = organic groups, CO = ketonic group O O C R 20 C H R R Functional Groups Ketones and Aldehydes 20 Formation of a bond using an sp2 hybrid orbital and a bond using the pz enables oxygen to form double bonds to carbon: O2pz C2pz H H : CO -bond Polarised -molecular orbital : H 120° H C O Functional Groups Ketones and Aldehydes • Carbon is positively polarised and oxygen negatively polarised • Carbonyls are best seen as: C + – O 20 Functional Groups Carboxylic acids R R = alkyl group, CO2H = carboxyl group CO2H O C OH 21 Functional Groups Carboxylic acids 21 • Why acidic? • In water they ionize partially H O O R C O H O Ka R C H O + H3 O (Hydronium ion) Carboxylate anion [RCO2-][H3O+] Ka = [RCO2H] pK a = -log K a Functional Groups Carboxylic acids 21 • Resonance: • Negative charge is on both oxygens O O R C R C O O R C O O Resonance hybrid QuickTime™ and a Animation decompressor are needed to see this picture.