Class Notes (810,488)
Canada (494,139)
Chemistry (378)
Felix Lee (12)

Topic 6

20 Pages
Unlock Document

Western University
Chemistry 2223B
Felix Lee

Chemistry 2213a  Fall 2012  Western University Topic 6. Benzene and Derivatives (Arenes) A. Structure  Benzene, C H 6 d6scovered by Michael Faraday in 1825, has 4 units of unsaturation. It is remarkably stable due to resonance.  Due to resonance, all carbon-carbon bonds are equivalent. All carbon and hydrogen atoms are in the same plane due to sp hybridization.  The structure is best described by molecular orbital theory. Th2 remaining p orbitals of the sp - hybridized carbon atoms overlap sideways to form a big molecular orbital that has a torus-shaped orbit on each side of the ring. Benzene and Derivatives  2  The carbon-carbon “double bonds” of the H Lewis structure are so stable that benzene H O no and compounds containing benzene rings 2 reaction don’t undergo the typical alkene reactions.  Is there a way to estimate how much stability resonance imparts to a benzene ring? Yes, by measuring the heat (H) of hydrogenation reactions of benzenes and alkenes.  Consider the addition of H to one C=C in cyclohexene: 2 Ni H 2 H = –120 kJ/mol  So, we expect benzene to be 3 × –120 = –360 kJ/mol, but the measured value is H = –208 kJ/mol. (Because of the stability of benzene, this reaction requires extreme conditions for hydrogenation to occur.) Ni 3 H2 H = –209 kJ/mol Benzene and Derivatives  3  Benzene is therefore 360 – 209 = 151 kJ/mol more stable than a cyclohexane ring containing 3 C=C is expected to be.  This increased stability due to resonance is called resonance energy. B. Aromaticity  Aromatic is the term used to describe cyclic compounds that contain double bonds but do not undergo the typical reactions of alkenes. Resonance stabilization is a central feature of aromaticity.  For a compound to be aromatic, it must meet three criteria: o Have a p orbital on each ring atom o Be planar or nearly planar o Have 2, 6, 10, 14, 18, etc.  electrons Benzene and Derivatives  4  Rings with heteroa2oms (atoms other than C or H) can be aromatic. The heteroatoms are sp -hybridized so that a lone pair can be placed in the remaining p orbital, which forms a molecular orbital with the p orbitals of the other atoms. (See “How To 9.1” in the text.) H O N furan pyrrole Benzene and Derivatives  5  Heterocyclic aromatic compounds are very common in bioorganic molecules. C. Nomenclature  Many aromatic compounds have pleasant smells, hence the term aromatic. For example, benzaldehyde (lab #5) smells like almond oil. However, many of them are toxic and carcinogenic (e.g. benzo[]pyrene in cigarette smoke). benzo[]pyrene Benzene and Derivatives  6  Many simple derivatives of benzene have common names. CH OH NH CHO 3 2 napthalene toluene phenol aniline benzaldehyde NO 2 COOH N CH 3 O 2 NO 2 benzoic acid pyridine TNT  When a benzene ring is named as a substituent, it is called a phenyl group. If the benzene ring is attached to2a CH , the entire group is a benzyl group. CH 2 CH3 CH 2 CH2 Cl phenyl group phenylethane benzyl group benzyl chloride Benzene and Derivatives  7  A special naming convention is used for disubstituted benzene compounds. o 1,2-substituted  ortho (o) 1,3-substituted  meta (m) o 1,4-substituted  para (p) CH 3 OH NH 2 Cl Br Cl m-bromotoluene p-chlorophenol o-chloroaniline  If there are multiple CH 3 substituents, the compound can be named similarly to Cl Br cycloalkanes. If the compound is named using a common name, the H2N substituent responsible for th1-chloro-4-ethyl- 4-bromo-3-methylaniline common name is always assigned position #1. 2-isopropylbenzene 5-amino-2-bromotoluene Benzene and Derivatives  8 D. Reactions of Aromatic Compounds 1. Ability to stabilize cations and anions  Although the aromatic ring is not very reactive, it is able to stabilize adjoining cations and anions by resonance, as previously seen. CR 2 CR 2 CR 2 CR 2 CR 2 CR 2 CR 2 CR 2 Benzene and Derivatives  9 2. Acidity of phenols  Resonance stabilization of the oxygen anion makes phenols much stronger acids than other alcohols. Added electron- withdrawing groups on the ring can make some phenols as acidic as carboxylic acids. OH OH O OH OH O2N NO 2 pKa = 15.9 10.0 4.0 4.8 3. Oxidation of benzylic carbon atoms  The  bonds of aromatic rings are not easily oxidized (as alkanes are), but carbon atoms directly attached to the benzene ring, termed benzylic carbons, are oxidized to carboxylic acids if they are bonded to one or more H. H 2rO 4r KMnO 4 CH COOH Benzene and Derivatives  10  Oxidation is not affected by other ring substituents. Tertiary (with respect to the other carbons besides the ring) benzylic carbons don’t oxidize because they don’t have any H. H N excess H N 2 KMnO 4 2 COOH (H3C)3C (H3C)3C COOH 4. Electrophilic aromatic substitution (EAS)  This is characteristic of all aromatic compounds E E+ H+  E is a strong electrophile, usually a cation. It can replace two or more H, but we’ll only look at monosu
More Less

Related notes for Chemistry 2223B

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.