Topic 5B Bonding in carbon compounds

9 sp3 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: l=1 l=0 ml = -1 0 1 Valence shell 2s2 2p2 2s 2p E p x y z s n=2

9 sp3 hybridization Promote one 2s electron into the vacant p-orbital. Combine (mix) all four orbitals to give four hybrid orbitals of equivalent energy: E

9 sp3 hybridization Each sp3 hybrid orbital has 25% “s” and 75% “p” character Each sp3 hybrid orbital looks like a distorted dumbell:

sp3 Hybridization Animation 9 sp3 Hybridization Animation Movie from Saunders General Chemistry CD-ROM 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:

10 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.

Sigma () bond formation 10 Sigma () bond formation Movie from Saunders General Chemistry CD-ROM

Other representations 10 Other representations Ball and stick Space Filling

Other representations 10 Other representations Space Filling Potential Energy Surface

C–C bond formation in Ethane 11 C–C bond formation in Ethane Sigma () bonds can be formed between two carbons by overlapping two sp3 hybrid orbitals. C H sp 3 - sp bond between carbons H H H C + C H H H

11 Ethane Ball and stick Space filling

Ethane 11 Ethane can spin about the C—C bond There is nearly free rotation:

Propane 11 Propane is formed by covalent bonding to two other carbons and eight hydrogens. Ball and stick Space filling

11 Propane Propane can rotate about both C—C bonds

Butane 11 7 Butane is formed by covalent bonding between four carbons and ten hydrogens. Ball and stick Space filling

11 7 Butane Butane can rotate about all three C—C bonds

12 Bonding to other atoms Alcohols are formed between sp3 hybridised carbon and oxygen: H H H H H C + O H C O H H sp 3 - sp valence bond between carbon and oxygen giving an alcohol

12 Bonding to other atoms Amines are formed between sp3 hybridised carbon and nitrogen: sp 3 - sp valence bond between carbon and nitrogen giving an amine N H C H H H C + N H H

sp2 Hybridization Double bond formation 13 sp2 Hybridization Double bond formation 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 H C C C O H H H Ethene (carbon sp2) Formaldehyde (carbon, oxygen sp2)

sp2 Hybridization 13 E combine 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. E 2s 2p E z y x 2p 2s combine 2p z 2sp 2

sp2 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. 3 sp 2 hybrid orbitals 120° 2s 2p y 2p x

Trigonal planar carbon 13 Trigonal planar carbon There are four electrons — one in each orbital Note that the 2pz orbital is unchanged and perpendicular to the plane of the hybrid system. C2p z sp 2 hybrid 120° An sp 2 hybridised carbon atom. 120° sp 2 hybrid 120° sp 2 hybrid

Pi () bonding Ethylene 13 Pi () bonding Ethylene Two sp2 carbons can form a covalentbond. Other hybrid orbitals covalently bond to four hydrogens. C2p z C2p z

14 Pi () bonding Ethene 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. C2p z H CH s bonds CC bond H CC p bond

Pi () bonding Ethene animation 14 Pi () bonding Ethene animation Movies from Saunders General Chemistry CD-ROM

Pi () bonding (theoretical approach) 14 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 energyorbital. p One *-molecular orbital E  * C2pz H One  -molecular

Ethylene 14 Because each carbon is trigonal planar, ethylene is a flat molecule with thickness due to the pi-electrons.

Ethylene 14 The pi-bond restricts rotation about the C=C bond. A little twisting is possible but it is essentially rigid.

15 Ethylene Geometry of ethylene: Other double bonded systems:

sp Hybridization Alkyne formation 15 sp Hybridization Alkyne formation 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: Ethyne (carbon sp)

sp Hybridization 15 E combine 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 z y x 2p 2s combine 2p 2s

sp Hybridization 15 Only the 2px combines with the 2s orbital. The two hybrid orbitals make angles of 180° to minimise electron repulsion between them. 180° 2s 2p x Two colinear sp hybrid orbitals

sp hybridised carbon 15 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. An sp hybridised carbon atom sp hybrid C2p y z

Triple bonding in Ethyne 16 Triple bonding in Ethyne Two sp hybridised carbons can form a covalentbond. Other hybrid orbitals covalently bond to two hydrogens. C2p z C2p z C2p y C2p y

Pi () bonding in Ethyne 16 Pi () bonding in Ethyne 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. C H y z C2p z CH s bond CC y

Ethyne (acetylene) 16 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.

Bond length—strength CC bonds 17 Bond length—strength CC bonds Summary: pm Bond length decreases from single to double to triple bond. Bond strength increases from single to double to triple bond.

Functional Groups Alcohols 18 Functional Groups Alcohols CH3OH Methanol

Functional Groups Alcohols 18 Functional Groups Alcohols R OH R = alkyl group, OH = hydroxyl group .. CH3CH2OH Ethanol

Functional Groups Alcohols 18 Functional Groups Alcohols Classification:

Functional Groups Amines 19 Functional Groups Amines In methylamine, sp3 nitrogen is covalently bonded to methyl and two hydrogens N CH 3 H C Methylamine (Methanamine)

Functional Groups Amines 19 Functional Groups Amines Classified on number of alkyl groups attached to nitrogen

Functional Groups Ketones and Aldehydes 20 Functional Groups Ketones and Aldehydes R CHO R = organic group, CHO = aldehyde group C H O R = organic groups, CO = ketonic group C R O CO

Functional Groups Ketones and Aldehydes 20 Functional Groups Ketones and Aldehydes Formation of a bond using an sp2 hybrid orbital and a  bond using the pz enables oxygen to form double bonds to carbon: : O2p z C2p CO s -bond H C O H Polarised p -molecular orbital 120°

Functional Groups Ketones and Aldehydes 20 Carbon is positively polarised and oxygen negatively polarised Carbonyls are best seen as:

Functional Groups Carboxylic acids 21 Functional Groups Carboxylic acids R CO2H R = alkyl group, CO2H = carboxyl group

Functional Groups Carboxylic acids 21 Functional Groups Carboxylic acids Why acidic? In water they ionize partially

Functional Groups Carboxylic acids 21 Functional Groups Carboxylic acids Resonance: Negative charge is on both oxygens