Aromatic Hydrocarbons

The benzene compounds

• So far, everything we’ve looked at was ALIPHATIC

• About 100 years ago, chemists became aware of a group of nice-smelling compounds-the AROMATICS

• Today we know that many aromatics stink

• 1825-the British chemist Faraday discovered a compd in whale oil

• Its formula was C6H6

• They knew that it wasn’t very reactive

• Alkanes unreactive

• Alkenes somewhat reactive

• Alkynes very reactive

• BUT, Benzene wasn’t reactive

• Also, benzene couldn’t be drawn as a saturated molecule

• How do you draw benzene with no double or triple bonds?

• 1865-German chemist name Kekule’ went home one day, sat down by his fireplace, and went to sleep

• He had a dream in which he was out walking in the country

• He saw a SNAKE!!

• He decided to outwalk it, but the SNAKE chased him!

• Up ahead Kekule’ saw a cliff

• He ran up to the edge and jumped off!!

• The SNAKE got to the edge, and looked down at him with an evil look

• He put his tail in his mouth and rolled down the hill!!

• Kekule’ woke up and had the answer to his C6H6 problem

• See structures on the board

• Skeptics asked, “how do you know which one it is?”

• X-ray diffraction analysis-allows us to measure bond length between carbons.

• The C-C bond is normally longer than the C=C

• BUT, in benzene, it was found that the C-C’s were shorter than normal and the C=C’s were longer

• The real structure of benzene is a hybrid of the 2-see board

• Resonance: the representation of a real structure by the average of 2 or more fictitious Lewis structures.

• The true structure-a composite of the contributing structures-is called the resonance hybrid.

• Resonance indicates stability.

• Why is resonance important?

• It helps chemists visualize the actual molecule as a hybrid.

• Magnetic Resonance Imaging

• -article of bad MRI

• Show a real one vs. x-ray

• Naming Aromatics is fun.

• Monosubstituted benzenes-NO2 -Br -Cl

-CH3 -NH2 -OH -COOH

• Be sure to know – Toluene– Phenol– Aniline– Benzoic Acid

Disubstituted Benzenes

• If a common name (toluene, aniline, phenol, benzoic acid) is present, use is at the parent name.

• These have special rules– Ortho or o (1,2)– Meta or m (1,3)– Para or p (1,4)

– OMP can only be used when 2 groups are on the ring!!– http://www.youtube.com/watch?

v=HPGE_GJkLJA&feature=related cheesy video

Polysubstituted Benzenes

• When a common name (toluene, aniline, benzoic acid, phenol) is present, use it as a parent name

The Phenyl Group

• Benzene can also be named as a side group.

• It’s not called “benzyl” as you would expect!

• Instead we call it “phenyl”– Phenyl is not the same as phenol

• Exs:– Phenyl chloride

– Diphenyl methane

– m-phenyl phenol

– Dichlorodiphenyltrichloroethane-moral story• DDT info 3 min video• http://videos.howstuffworks.com/hsw/15861-aggrav

ation-of-accumulation-ddt-and-bald-eagles-video.htm

Practice/review drawing these!

• 1. o-dichlorobenzene• 2. m-bromophenol• 3. p-xylene• 4. 1,2-difluoro-3-iodobenzene• 5. 2,3-dinitroaniline• 6. o-phenyltoluene• 7. m-dibenzylbenzene• 8. phenyl bromide• 9. o-phenyl phenol

Fused Aromatics

• Some compounds have 2 or more benzene rings fused together

• Exs:– Naphthalene

• In mothballs, a solid at room temperature

Cig smoke, car exhaust, and burned food all contain fused aromatics

So does chimney soot-testicular cancerMary Poppins videohttp://www.youtube.com/watch?v=yu23HHmOG48Same guyshttp://www.youtube.com/watch?v=xRYU4cqUAUs

More on DDT

• It’s colorless, odorless, tasteless• Banned in US but still used in some

underdeveloped countries• Has saved lives by killing disease-carrying

insects (lice, ticks, mosquitoes)• Worst thing about it is stability• Biological magnification• How DDT kills birds

Misc Interesting Stuff

• PABA (p-aminobenzoic acid) in some sunscreens• Various forms of sunscreen have been around for

thousands of years. Ancient Egyptians used jasmine as a sunscreen, and ancient Greeks rubbed olive oil on their skin for protection.

• chemical sunscreens (avobenzone,benzophenone) absorb the sun's harmful UV rays while physical sunscreens, or sunblocks (zinc oxide, titanium dioxide), reflect them.

• Broad spectrum sunscreens contain both types (above)• SPF-skin protection factor-doesn’t account for wind, angle

of sun, elevation, or reflection• Min recommended SPF is 15

Natural sunscreen?Melanin-a dark-colored skin pigment that offers natural protection

from the sunIt’s why we’re all colored people! Well, most of us Albinos have a birth defect they don’t have melanin

http://videos.howstuffworks.com/discovery/30903-why-tell-me-why-why-skin-color-varies-video.htm

Makes up hair color, skin color, and eye colorAlso makes up freckles

Bad stuff

• Tanning beds give off same kind of radiation as sun

Miss Maryland and melanoma-3 minhttp://videos.howstuffworks.com/multivu/1844-melanoma-video.htmShould you get a tan? 14 minhttp://videos.howstuffworks.com/planet-green/37339-g-word-organic-spray-tan-video.htmSunburns are bad 1 minhttp://videos.howstuffworks.com/howstuffworks/131-how-sunburns-work-video.htm

Good stuff

• Our body needs some aromatics-we get them from our diet ex: vitamins

• You need sunlight-1.5 min videos-see which one is better

• http://videos.howstuffworks.com/sciencentral/4149-how-not-getting-enough-sun-can-be-harmful-video.htm

• http://videos.howstuffworks.com/discovery-health/32522-cleveland-clinic-2008-vitamin-d-video.htm