Benzene Flashcards
(60 cards)
what type of compound is benzene?
an aromatic compound
what does aromatic mean?
some or all of the carbon atoms are in a benzene ring
when was benzene first isolated?
in 1825 by Michael faraday, who deduced that its empirical formula was CH
What do we call organic compounds that do not contain a benzene ring?
aliphatic (i.e. straight or branched chains or cyclic)
what happened in 1834 to the benzene?
In 1834, the German chemist Eilhard Mitscherlich determined that benzene’s Mr was 78, and so its molecular formula was therefore C6H6.
Benzene in 1865
In 1865, the French chemist Friederich Kekulé proposed a cyclic structure for benzene consisting of alternating single and double bonds.
1931 benzene
However it was not until 1931 that benzene’s structure was fully resolved.
what are the three types of experimental evidence that shows Kekulé structure to be incorrect?
- bond lengths
- enthalpy of hydrogenation
- lack of reactivity
what does the kekule structure show in terms of bond length?
The Kekulé structure shows alternating single and double bonds.
( bond lengths C-C 0.154 nm, C=C 0.134 nm)
what disapproves of the bond length of kekule?
using x-ray crystallography, all the C-C bonds were found to be the same length (0.140 nm)
what is the stability of benzene and why?
Benzene is a more stable molecule than would be expected if it had 3 “localised” double bonds, due to the delocalised pi system
what evidence on enthalpy of hydrogenation showed Kekule’s to be incorrect?
If benzene is simply cyclohexa-1,3,5-triene, having three localised C=C double bonds, then you would expect the hydrogenation enthalpy for 3 C=C double bonds, would be three times that of hydrogenating 1 C=C double bond.
What would the predicted value for hydrogenating the kekule structure be?
Predicted value =
3 x –120 =
-360kJmol-1
What is the actual value for hydrogenating the kekule structure be?
Actual value = -208kJmol-1
what does the actual value mean for benzene?
This shows that benzene is more stable due to the delocalised π- electron ring structure by 152 kJmol-1
what evidence on reactivity showed Kekule’s to be incorrect?
The reactivity of benzene also suggested evidence that the cyclohexatriene structure was not correct. Benzene does not undergo addition reactions with bromine, unlike alkenes, due to the stability of the delocalised π-electron ring system.
what are examples of reactions that benzene does not undergo?
Benzene does not undergo electrophilic addition with bromine water.
what is the accepted structure of benzene?
delocalised ring structure
what is the structure of benzene description?
P orbitals –> π-bond above and below the C’s
Delocalised p-electron ring above and below the planar carbon structure
the bonding in benzene is described as?
- p-orbitals overlap side-on to form p-electron ring system. (p bond delocalised over all 6 carbons)
- 6 delocalised electrons in the p-electron ring.
- the C-C and C-H single bonds are sigma, s bonds
- The molecule is planar, C-C-H bond angles are 120o
The delocalised π-electron ring in benzene gives the structure ADDED STABILITY.
How does this affect breaking a bond in benzene?
More energy is needed to break the bonds
What type of bond breaks during an addition reaction in benzene, what does this mean?
a pi-bond
This means that benzene is resistant to addition reactions.
what formula can benzene be represented by?
C6H6 , although for this course we will stick to the first 2
Some groups will be shown as prefixes to benzene. These include:
- short alkyl chains
Methylbenzene, C6H5CH3
