Introduction 1. Benzene has the formula _____ and was discovered in 1825 by _______ _______. This compound and others like it were called ____________ because of its pleasant odor. Today, the term aromatic refers to the unusual ____________ these compounds possess. C6H6C6H6 Michael Faraday aromatic stability
The Structure & Properties of Benzene 1. Draw the structure of benzene Kekulé proposed in 1866.
2. Experimental evidence shows that all the carbon-carbon bonds in benzene have the same bond length. Why is this in disagreement with Kekulé's structure? Double bonds are expected to be shorter than single bonds. Kekulé's structure should have alternating short and long bonds.
Draw a better representation of the benzene structure. The bond order between carbon atoms is _______, the bond angles are _______, and the carbon atoms are _____-hybridized. = 1½ 120° sp 2
3. The benzene molecule is flat, with the unhybridized _____ orbitals extending above and below the plane of carbon atoms. Draw the benzene ring showing the p orbitals overlapping to form a continuous ring of orbitals above and below the plane of carbon atoms. p sp 2 framework p orbital circle of delocalized electrons
4. The double bonds in benzene are said to be _______________, which means that they are separated by one single bond and overlap their p orbitals in the _____ bonds. Draw examples of conjugated and non-conjugated double bonds. conjugated non-conjugated
5. Based upon what you've learned about benzene, are aromatic compounds cyclic or acyclic? Are they planar or nonplanar? Do they contain conjugated or non-conjugated double bonds? Do they have or lack resonance stability?
6. Draw the structure of cyclooctatetraene. Using the information in number 5, is this compound expected to be aromatic? Yes, it’s expected to be aromatic because it appears to... be cyclic be planar be conjugated have resonance stability
Construct a model of cyclooctatetraene. Is this consistent with your prediction of whether or not cyclooctatetraene is aromatic? No Why or why not? In this view, cyclooctatetraene appears to be planar, and could be aromatic. This view shows that cyclooctatetraene is nonplanar, therefore it cannot be aromatic.
Cyclooctatetraene... is not planar. is non-conjugated. has no resonance structures. is not aromatic. We need a better set of requirements for a molecule to be aromatic.
Aromatic, Antiaromatic, & Nonaromatic Compounds 1. A compound is Aromatic if… It is _______ with _________ pi bonds. All the atoms in the ring are _____- or _____-hybridized. It is _________ (or nearly) so that it forms a continuous, overlapping ring of parallel _____ orbitals. It has _______ energy due to delocalization of _____ electrons. cyclicconjugated sp sp 2 planar p lower
2. A compound is Antiaromatic if… Numbers , , and are True, but is False. Cyclobutadiene is antiaromatic. Why? Remember what the polygon rule told us about the stability of benzene vs. cyclobutadiene. The polygon rule shows that cyclobutadiene is expected to be unstable. There is no resonance stability.
3. A compound is Nonaromatic if… Neither the criteria described in number 1 nor number 2 apply. Cyclooctatetraene is nonaromatic. Why? Consider whether the four criteria in number 1 are true or false. cyclic, but non-conjugated sp 2 -hybridized carbon atoms nonplanar (poor overlap of p orbitals) higher energy (polygon rule)
Hückel’s Rule 1. Another way to predict whether a compound is aromatic or antiaromatic is Hückel's Rule. Before we can apply this rule, the compound must have a _______________ ring of overlapping _____ orbitals, usually in a _________ arrangement. Once these conditions are met, we can use Hückel's Rule. continuous p planar
2. Hückel's Rule says: If the number of pi electrons in the cyclic system is… 4N + 2, the system is _________________. 4N, the system is _________________. Where N is an integer. aromatic antiaromatic
4. Fill in the table below. 614N + 2 Aromatic 414N Anti- aromatic 824N Anti- aromatic if planar* *COT is nonplanar = nonaromatic (Hückel’s rule doesn’t apply).
5. Fill in the table below. 1024N + 2 Aromatic 824N Anti- aromatic 824N Non- aromatic* *There is not a continuous ring of overlapping p orbitals (Hückel’s rule doesn’t apply).
Nomenclature of Benzene Derivatives 1. Although benzene is a stable aromatic compound, it doesn't mean it's unreactive. Benzene undergoes many reactions to form useful derivatives. Some of the more commonly used derivatives, shown below, use common names instead of IUPAC names. For each compound below, give the common name and the IUPAC name.
2. Some benzene derivatives are named by combining the substituent name with the word "benzene." Name the following compounds. ethylbenzene isopropylbenzene t-butylbenzenenitrobenzene
3. When benzene loses one H atom and becomes a substituent, it forms the C 6 H 5 — group and is named _________. When toluene loses one methyl H atom and becomes a substituent, it forms the C 6 H 5 CH 2 — group and is named _________. Name the following compounds. phenyl benzyl diphenyl etherbenzyl chloridebenzyl alcohol
4. Disubstituted benzenes are given common names by using the prefixes ortho-, meta-, and para- (abbreviated _____, _____, and _____) to show the relative position of the two substituents on the benzene ring. Numbers are used by the IUPAC system. For example, the numbers _____ indicate ortho-, _____ indicate meta-, and _____ indicate para-. o-o-m-m-p-p- 1,2- 1,3- 1,4-
Identify the substitution pattern in each example below. ortho- or o-meta- or m-para- or p- 1,2-1,3-1,4-
5. When there are two methyl groups on the benzene ring, the molecule is given the common name xylene. For each compound below, give the common name and the IUPAC name. o-xylenem-xylenep-xylene 1,2-xylene1,3-xylene1,4-xylene
6. For each compound below, give the common name and the IUPAC name. o-dibromobenzene 1,2-dibromobenzene m-bromochlorobenzene 1-bromo-3-chlorobenzene P-fluoronitrobenzene 1-fluoro-4-nitrobenzene o-chloroethylbenzene 1-chloro-2-ethylbenzene m-fluorotoluene 3-fluorotoluene p-chlorophenol 4-chlorophenol
7. When there are three or more substituents on the benzene ring, numbers are used to show their relative positions. One substituent is often used to form the base name and this substituent is assigned number 1. The other substituents are numbered in the direction that gives the lowest possible set of numbers. Name the following compounds. 1,3,5-tribromobenzene 2-bromo-4-chloro- 1-nitrobenzene 4-bromo-1-chloro- 2-nitrobenzene 1 3 5 1 2 4 1 2 4
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