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chem 104 Lecture 1
Organic Chemistry
32
Chemistry
Undergraduate 1
02/22/2013

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Term
how the the carbons in benzene hybridised?
Definition
sp2
Term
what is the bond angle of a carbon that is sp3 hybridised? what is the shape of this molecule?
Definition

109.5 degrees

shape is tetrahedral

Term

true or false?

saturated carbon atoms are sp3 hybridised?

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

four

four

Term
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?
Definition

bond angle 120 degrees

sp2 hybridisation eg. alkenes like propene

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

true or false?

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

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

bond angle 120 degrees

planar geometry

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

true or false?

aromatic compounds undergo electrophilic addition like alkenes?

Definition
false, they undergo electrophilic substitution
Term

complete the definition of aromatic compounds:-

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

Definition

unsaturated 

resonance

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

requirements for aromaticty

 

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

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

14 and 18 are aromatic and planar

16 annulene is non planar and non aromatic

Term
can compounds containing fused benzene rings still be aromatic?
Definition

yes silly course they can!

eg. Naphthalene

eg. Phenanthrene

eg.Anthracene

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

sp2 hybridised.

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

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

in pyridine the lp is NOT part of the pi system

in Pyrrole the lp IS part of the pi system

Term
in a furan ring oxygen is present. oxygen has 2 lp of electrons. how many participate in the pi system?
Definition
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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