CHM 456Organic Chemistry 1

1.0 Carbon Compoundsand Chemical Bonds

1.0 Carbon Compoundsand Chemical Bonds

Objectives

• 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)

http://www.middleschoolchemistry.com/multimedia/chapter4/lesson4

- attracted to +

Why do bonds form?• Due ultimately to EntropyEntropy (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

entropyentropy 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 entropyentropy by the products formed is greater than the heat (enthalpy) loss of the surroundings (Heat energy, exothermic) and Entropy (Energy of ‘disorder’) increases

http://www.webchem.net/notes/chemical_bonding/covalent_bonding.htm

Repulsive EnergyMORSE CURVE

Attraction Energy

0

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/Kinetics-online/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 bonds1) 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 bondsdative covalent bonds or

coordinate bondscoordinate 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

Electronegativity

Electronegativity• 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 forcesIntermolecular forcesNEED TO KNOW ABOUT ELECTRONEGATIVITY NEED TO KNOW ABOUT ELECTRONEGATIVITY

AND MOLECULAR SHAPEAND 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

1/3Hydrogen 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 FFound in: Water, HF, ammonia, alcohols, carboxylic acids, amines, water with propanone, water with ethanal

Water molecules

http://alevelnotes.com/?id=135

2/3Dipole-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/books/introductory-chemistry/section_14_01.html

3/3Induced 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!

• http://www.youtube.com/watch?v=S8QsLUO_tgQ&NR=1&feature=endscreenDr Paul McCord RELATIVE LINKRELATIVE LINK

Summary Videos = http://www.youtube.com/watch?v=dQ33TVQCfyY RELATIVE LINKRELATIVE LINK

Note specific animation of induced-induced dipoles = from 4m:39s onwards

• Nice (but brief) website on molecular geometry:

http://www.elmhurst.edu/~chm/vchembook/208introgeom.html

spdf notation

spdf notation

http://titanchem1.blogspot.com/

HybridizationHybridization

http://89.97.218.226/web1/letiochem/lezioni/sigma-pigreco.htm

Sigma () bonding

sigma bonds have a plane of symmetry along an internuclear axis.

pi (π) bonding

http://89.97.218.226/web1/letiochem/lezioni/sigma-pigreco.htm

Nodal pane

Double bonds

http://chempaths.chemeddl.org/services/chempaths/?q=book/General%20Chemistry%20Textbook/Further%20Aspects%20of%20Covalent%20Bonding/1345/sigma-and-pi-bonds

WEAKER ATTRACTION (=weaker bond =more reactive bond) of e- in pi-bond to the nucleus as e- are further away!!!

C≡C (triple bond)

http://chemistrypractice.blogspot.com/2010/10/multiple-bond.html

Other representations…

http://www.wag.caltech.edu/home/jsu/Thesis/node46.htmlhttp://www.brynmawr.edu/chemistry/Chem/Chem103Lkdl/consider/consider5.htm

Other representations…

http://www.chem.ufl.edu/~itl/4412_aa/ccbond.html

http://www.chem.ufl.edu/~itl/2045/lectures/lec_15.html

Hybridization

Hybridization

http://iverson.cm.utexas.edu/courses/310M/Handouts/Handoutsfl05/MOVBTheory.html

Hybridization

• http://www.youtube.com/watch?v=g1fGXDRxS6k VIDEO: Hybridization (Low).flv (US)

• http://www.youtube.com/watch?v=R-sxDXnOgnk 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/lectures/lec_15.html for a good discussion of this.

• and also

Bonding and Anti-bonding

http://www.chem.ufl.edu/~itl/2045/lectures/lec_15.html

http://bouman.chem.georgetown.edu/S00/exam1/exam1.htm

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 LINKRELATIVE LINK)

•Resonance and Predicting Chemical Reactivity (http://www.youtube.com/watch?v=oOBNJCNnFE8) Higher Level. 29mins