Aromatic Hydrocarbons PDF

AROMATIC HYDROCARB ONS Aromatic Compounds – HC also known as arenes which characteristics which are different from Aliphatic compounds. - Greek word “ Aroma” means pleasant smelling.. Arenes - A compound containing one or more benzene rings. Aryl gro...

AROMATIC HYDROCARB ONS Aromatic Compounds – HC also known as arenes which characteristics which are different from Aliphatic compounds. - Greek word “ Aroma” means pleasant smelling.. Arenes - A compound containing one or more benzene rings. Aryl group – a group derived from an arene by removal of a H atom from arene. Benzene (C6H6) – simplest aromatic compounds - Cyclic unsaturated hydrocarbons with delocalized electrons - discovered by Michael Faraday in 1825. The simplest aromatic hydrocarbon is benzene (C6H6) H H H H H H H H H OR… H H H H H H H H H Example of Aromatic Hydrocarbons Properties of Benzene  an important compound in both the chemical industry and laboratory.  poisonous if ingested in liquid form.  Vapor form is toxic can be absorbed by breathing or through the skin.  Long term inhalation can cause liver damage or cancer Chemistry of Benzene 1. Compound with few hydrogen atoms for its’ 6 C molecule. 2. Unsaturated but doesn’t behave like an alkene ( 3 alternating double bonds) 3. Does not react with the following Br2, HBr, H2O and H2SO4 4. Kekule structure: Benzene is a resonance hybrid structure is confirmed by X-ray diffraction research. Naming Aromatic Compounds A. Monosubstituted Derivatives of Benzene Benzene is the “first and simplest” aromatic compound and many monosubstituted derivatives of benzene are named systematically by adding the name of the substituent to “benzene” which is the parent: For examples: Phenyl group (C6H5) – substituent group derived by loss of a hydrogen from benzene. If the substituent is an alkyl chain with more carbon atoms than benzene, then benzene can be treated as a substituent. The ring, in this case, is called a phenyl group just like the methyl, ethyl, etc. The phenyl group is often abbreviated as “Ph”. B. Two substituents - when two substituents occur on a benzene ring, three possible isomers are possible, Geometric Isomerism in Aromatics Cl H Cl ortho (o-) = two adjacent substituents o-dichlorobenzene H H Cl H H H meta (m-) = one carbon between substituents H Cl m-dichlorobenzene Cl H H H para (p-) = two carbons between substituents p-dichlorobenzene H H Cl  Disubstituted Derivatives of Benzene Some disubstituted benzene rings also have common names, and the first thing here is to know the relative positions of ortho, meta, and para: C. In general, to name an aromatic compound, you can follow these steps: a. Identify and name the parent. If it is not one of the common names, then use benzene. b. Identify and name the substituents. c. Number the ring to give the substituents the smallest possible number. Put the substituents alphabetically followed by the parent name. For example, in ortho-dibromobenzene, numbering from the top Br goes clockwise to have a 1,2, instead of 1,6 locant order: We do the same way if there are more substituents: If the numbering does not make a difference, then give a smaller locant to the alphabetical priority: If there are more than two groups, and numbering doesn’t make a difference, start from the alphabetical priority and number the ring toward the next alphabetical priority: Two common names are possible. For example phenol and aniline, we need to go based on the priority of the groups. In this case, we are comparing the amine and alcohol, and since OH stands above NH2 in the priority chart, the parent chain should be an alcohol (phenol) rather than aniline. The Similarities and differences between the Phenols and Alcohols 1. Both alcohols and phenols are flammable. 2. Dehydration is a reaction of alcohols but not of phenols; phenols cannot be dehydrated. 3. Both 1o and 2o alcohols and phenols do not react with the oxidizing agents that cause 1o and 2o alcohol oxidation Phenols can be oxidized by stronger oxidizing agents. 4. Both alcohols and phenols undergo halogenation in which the hydroxyl group is replaced by a halogen atom in a substitution reaction. Practice 1. Give an acceptable IUPAC name for each molecule according to the nomenclature of aromatic compounds. For some compounds, there may be more than one acceptable name.