CHM 456
Organic Chemistry 1
1.0 Carbon Compounds
and Chemical Bonds
Objectives
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Understand what a bond is
Why they form
Lewis strictures (assigning e- in bonds)
Hybridization (mixing of orbitals)
Molecular shapes
Types of bonds (covalent, ionic, dative)
Formal Charge (calculatione)
Resonance (spreading of charge)
What is a bond?
• A electrostatic attraction between electrons
and the protons inside of two (or more)
nuclei.
• Hydrogen (the simplest case)
- attracted to +
http://www.middleschoolchemistry.com/multimedia/chapter4/lesson4
Why do bonds form?
• Due ultimately to Entropy (symbol ‘S’)
• Think of entropy as the disordered distribution
of energy.
• A process can only occur if it leads to a overall
increase in entropy. Many bond forming
reactions lead to an increase the entropy.
Why do bonds form?
• A chemical reaction increases the amount of
entropy usually by giving off heat energy
(enthalpy) (exothermic reaction), which then
randomly disperses throughout the
surroundings (i.e. the universe).
• Sometimes reactions are endothermic, so
increase in entropy by the products formed is
greater than the heat (enthalpy) loss of the
surroundings (Heat energy, exothermic) and
Entropy (Energy of ‘disorder’) increases
MORSE CURVE
Repulsive Energy
0
Attraction Energy
http://www.webchem.net/notes/chemical_bonding/covalent_bonding.htm
How quickly do bonds form
• The speed (rate) at which a reaction occurs
depends on a few factors, perhaps the most
important is the reactions ACTIVATION
ENERGY
• The minimum amount of
energy necessary for a
chemical reaction to
occur. It acts as a ‘barrier’,
preventing reaction.
http://www.chem.msu.su/eng/teaching/Kineticsonline/chapter6e_ad.html
Reaction rate, Activation Energy.
• A HIGHER the activation energy (symbol Ea) ,
more of a barrier, the slower the rate of
reaction.
• The LOWER the activation energy, less of a
barrier, the quicker the reaction will occur.
Ea ↑, then rate ↓
Types of bonds
1) Formation of covalent bonds:
A pair of e- is shared between two atoms. Each atom
contributing one electron.
– A subcategory of this is dative covalent bonds or
coordinate bonds. A pair of e- is shared between two
atoms, both e- come from just one atom
The bond is due to the mutual(joint) attraction
for the shared e- pair between the nuclei s
responsible for the 2) Ionic bonds. Electrons are
transferred from one atom to another.
Types of bonds
2) Formation of Ionic bonds. Electrons are
transferred from one atom to another.
The bond is due to the electrostatic attraction
between the resulting positive and negative
ions.
Dual character
• In reality many substances show a degree of
each type of bonding. E.g. CaI2 is ionic with
partially covalent character.
• AlBr3 is covalent with partially ionic character
This is due to polarisation
http://www.ibchem.com/IB/ibnotes/full/bon_htm/4.2.htm
Dot cross diagrams
http://2a2science.blogspot.com/2011_02_10_archive.html
http://www.ibchem.com/IB/ibnotes/full/bon_htm/4.2.htm
• VERY IMPORTANT
• Applies to covalent bonds
• Causes the polarisation of covalent bonds
http://www.pta210.info/Handouts/Electronegativity%20Table.gif
Molecular SHAPE (VSEPR)
• VSEPR in Chemistry (Low).flv (relative link)
• VSEPR Theory (Low).flv (video 2)
http://www.masterorganicchemistry.com/2010/09/24/how-to-calculate-formal-charge/
Intermolecular forces
NEED TO KNOW ABOUT ELECTRONEGATIVITY
AND MOLECULAR SHAPE
• These are attractions between (or ‘in
between’) DIFFERENT independent species,
e.g. water molecules, or Br2 molecules or CO2
molecules.
• The are NOT the same as the BONDS that are
responsible for the formation of molecules (
or ion pairs).
Intermolecular foeces.
Van der Waals forces
1/3
Hydrogen bond.
A H atom is bonded to an N, O or F atom and
interacts with a lone pair of electrons on a
different N, O or F
Found in: Water, HF, ammonia, alcohols,
carboxylic acids, amines, water with propanone,
water with ethanal
Water molecules
http://alevelnotes.com/?id=135
2/3
Dipole-dipole forces.
A polarised part of a molecule attracted to a
different and oppositely charged polarised part
of a molecule. Need to know about
ELECTRONEGATIVITY ! and molecular shape
Adapted from
http://flatworldknowledge.lardbucket.org/boo
ks/introductory-chemistry/section_14_01.html
3/3
Induced dipole-induced dipole forces
(London dispesion forces, or temporary dipole –
temporary dipole)
Electrons clouds distort randomly and at one
instant become polarised which can cause
polarisation in adjacent molecules.
http://www.dlt.ncssm.edu/tiger/chem3.htm
IMF videos - Watch these videos!
Summary Videos =
http://www.youtube.com/watch?v=dQ33T
VQCfyY RELATIVE LINK
Note specific animation of induced-induced
dipoles = from 4m:39s onwards
• http://www.youtube.com/watch?v=S8QsLUO_
tgQ&NR=1&feature=endscreen
Dr Paul McCord RELATIVE LINK
• Nice (but brief) website on molecular
geometry:
http://www.elmhurst.edu/~chm/vchembook/20
8introgeom.html
spdf notation
spdf notation
http://titanchem1.blogspot.com/
Hybridization
Sigma (s) bonding
sigma bonds have a plane of symmetry along an internuclear axis.
http://89.97.218.226/web1/letiochem/lezioni/sigma-pigreco.htm
pi (π) bonding
Nodal pane
http://89.97.218.226/web1/letiochem/lezioni/sigma-pigreco.htm
Double bonds
WEAKER ATTRACTION
(=weaker bond =more
reactive bond) of e- in
pi-bond to the nucleus
as e- are further away!!!
http://chempaths.chemeddl.org/services/chempaths/?q=book/General%20Chemistry%20Textbook/Further%20Aspects%20of%20Covalent%20Bondi
ng/1345/sigma-and-pi-bonds
C≡C (triple bond)
http://chemistrypractice.blogspot.com/2010/10/multiple-bond.html
Other representations…
http://www.brynmawr.edu/chemistry/
Chem/Chem103Lkdl/consider/consider5.htm
http://www.wag.caltech.edu/home/jsu/Thesis/node46.html
Other representations…
http://www.chem.ufl.edu/~itl/4412_aa/ccbon
d.html
http://www.chem.ufl.edu/~itl/2045/lectures/l
ec_15.html
Hybridization
Hybridization
http://iverson.cm.utexas.edu/courses/310M/Handouts/Handoutsfl05/MOVBTheory.html
Hybridization
• http://www.youtube.com/watch?v=g1fGXDRx
S6k VIDEO: Hybridization (Low).flv (US)
• http://www.youtube.com/watch?v=RsxDXnOgnk VIDEO: Hybridisation
Hybridization (sp, sp2, sp3) (Low).flv
(compilation)
Antibonding
• When you atomic orbitals coming together (in
a chemical reaction) to form molecular
orbitals, you produce bonding molecular
orbitals AND anti-bonding molecular orbitals.
• Read:http://www.chem.ufl.edu/~itl/2045/lect
ures/lec_15.html for a good discussion of this.
• and also
Bonding and Anti-bonding
http://www.chem.ufl.edu/~itl/2045/lectures/l
ec_15.html
ATOMIC ORBITALS  MOLECULAR ORBITALS
Formal Charge
• Describes the localised charge (on atoms) in a
molecule. It’s a useful tool*.
• For an atom. Consider it’s electron environment.
Then calculate the formal charge
Formal charge = [# of valence electrons] –
[electrons in lone pairs + 1/2 the number of
bonding electrons]
VIDEO: Formal Charges (Low).flv
Formal Charge
Alternatively (slight ‘simplification’ of the
formula)…
• Formal Charge = [# of valence electrons on
atom] – [non-bonded electrons + number of
bonds].
Resonance
• A HUMAN IDEA / TOOL / METHOD to
describe/show/predict the delocalisation of
change.
• Delocalisation = spreading out of electrons!
• Delocalisation occurs to lower a molecules (or
molecular ions) energy, chiefly by distributing
negative charge to positive areas and lowering
repulsion.
http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/Resonance
Resonance
http://chemwiki.ucdavis.edu/Theoretical_Chemistry/
Chemical_Bonding/Resonance << GOOD SITE!
Resonance contd.
Resonance video: Drawing Lewis Structures Resonance
Structures - Chemistry Tutorial
http://www.youtube.com/watch?v=MWDL5WCZBzE
(Low).flv (RELATIVE LINK)
• Resonance and Predicting Chemical Reactivity
(http://www.youtube.com/watch?v=oOBNJCNnFE8
) Higher Level. 29mins