chem 104 Lecture 1

how the the carbons in benzene hybridised?
what is the bond angle of a carbon that is sp3 hybridised? what is the shape of this molecule?

109.5 degrees

shape is tetrahedral

true or false?

saturated carbon atoms are sp3 hybridised?

this is true Px Py and Pz orbitals are in use
a carbon has …. valence orbitals and …. valence electrons



any carbon forming 1 double bond and 2 single bonds is trigonal planar but what is the bond angle and what is the carbon hybridisation?

bond angle 120 degrees

sp2 hybridisation eg. alkenes like propene

which p orbital is involved in making the pi (double bond) in sp2 hybrids?
it is the last one Pz orbital. this is the unhybridised orbital.
in sp2 hybrids what are single bonds also known as?
sigma bonds
which type of bonds are formed from HEAD ON overlap of two sp2 hybrid orbitals?
sigman (single) bonds

true or false?

pi bonds are formed from SIDE ON overlap of the two Py orbitals?

False! it is from the side on over lap of 2 Pz orbitals
the 2 electrons in a c-c pi bond are less stable. does this mean they are higher or lower in energy?
they are higer in energy and more accesible that electrons in sigma bonds. this makes alkenes behave in an electron rich way
what do you call something with alternating double bonds that overlap to form an extended pi system?
conjugated alkenes
what reactions do alkenes and conjugated alkenes undergo?
electrophilic addition
who first studied benzene?
michael faraday in 1825
what are the bond angles in benzene and what is its geometry?

bond angle 120 degrees

planar geometry

benezene is unuasually stable due to which property?
due to its resonance energy of approx 150KJ/mol
for any chemical species if more than one stable lewis structure can be drawn, what does this imply about its stability?
that it will be more stable that otherwise expected
can the structure of benzene be represented by one localised lewis structure?
no. it has resonance thus needs more than one lewis structure to show its structure.
the heat of hydrogenation of benzene is less than three times that of cyclohexene. what is this extra thermodynamic stability called?
its resonance energy of approximatley 150kj/mol

true or false?

aromatic compounds undergo electrophilic addition like alkenes?

false, they undergo electrophilic substitution

complete the definition of aromatic compounds:-

to be aromatic a compound must be ………………… and stabilised by a significant …………… energy



what does the pi electron density form above and below the molecular plane of benzene?
a torus (a revolving circle in 3 dimensional space)

requirements for aromaticty


a continous ……… cyclic pi system is a necessary condition. but this is still not sufficient

to be aromatic every atom in the ring must be sp2 hybridised. why?
so it can contribute a P orbital to the cyclic pi system
can a compound be aromatic if the ring atoms are not all lying in one plane?
no it cannot. this is one of Huckels criteria for aromaticity that the ring atoms must all lie in ONE plane
to be aromatic the pi system must contain a total of 4n +2 pi electrons. what does n stand for?
it is an integer (whole number)
what is the difference betwen 14,16 and 18 Annulene?

14 and 18 are aromatic and planar

16 annulene is non planar and non aromatic

can compounds containing fused benzene rings still be aromatic?

yes silly course they can!

eg. Naphthalene

eg. Phenanthrene


in fused rings are the pi electrons that are common to both rings shared or not shared?
they are shared. each ring is considered to have an aromatic sextet by sharing these pi electrons
what is pyridine an example of ?
a heterocyclic aromatic ring.
in pyridine what is the hybridisation of the Nitrogen?

sp2 hybridised.

its lone pair of electrons is in an sp2 orbital and NOT part of the pi system.

what is the difference between the nitrogen in pyridine and Pyrrole?

in pyridine the lp is NOT part of the pi system

in Pyrrole the lp IS part of the pi system

in a furan ring oxygen is present. oxygen has 2 lp of electrons. how many participate in the pi system?
one pair is part of the pi system. the other pair in in an sp2 hybrid orbital and not part of the pi system

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