Review and Introduction to Organic Chemistry

Introduction • In 1770 Tobern Bergman (Sweeden) was the first to express this difference between “organic” and “inorganic” substances ► Organic chemistry came to mean the chemistry of compounds from living organisms • Antoine Lavoisier (France) discovered that all organic materials are composed of Carbon in combination with other elements (H, N and O are the most common)

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Vitalism
• It was originally thought organic compounds could be made only by living things by intervention of a “vital force” • In 1816 Michel Chevreul (France) hydrolyzed soap into fatty acids H3O+ → “fatty acids”

soap

• Fredrich Wöhler disproved vitalism in 1828 by making the organic compound urea from the inorganic salt ammonium cyanate by evaporation:

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After this, many organic compounds were synthesized in the lab. Today, Organic Chemistry is the chemistry of the compounds of carbon. The human body is largely composed of organic compounds Organic chemistry plays a central role in medicine, bioengineering etc.

• Chemical Bonds: The Octet Rule
– Octet Rule
• Atoms form bonds to produce the electron configuration of a noble gas (because the electronic configuration of noble gases is particularly stable) • For most atoms of interest this means achieving a valence shell configuration of 8 electrons corresponding to that of the nearest noble gas • Atoms close to helium achieve a valence shell configuration of 2 electrons • Atoms can form either ionic or covalent bonds to satisfy the octet rule

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– Ionic Bonds
• When ionic bonds are formed atoms gain or lose electrons to achieve the electronic configuration of the nearest noble gas
– In the process the atoms become ionic

• The resulting oppositely charged ions attract and form ionic bonds • This generally happens between atoms of widely different electronegativities • Example

– Electronegativity
• Electronegativity is the ability of an atom to attract electrons • It increases from left to right and from bottom to top in the periodic table (noble gases excluded)
– Fluorine is the most electronegative atom and can stabilize excess electron density the best

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– Covalent Bonds
• Covalent bonds occur between atoms of similar electronegativity (close to each other in the periodic table) • Atoms achieve octets by sharing of valence electrons • Molecules result from this covalent bonding • Valence electrons can be indicated by dots (electron-dot formula or Lewis structures) but this is time-consuming • The usual way to indicate the two electrons in a bond is to use a line (one line = two electrons)

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• Polar Covalent Bonds
– Polar covalent bonds occur when a covalent bond is formed between two atoms of differing electronegativities
• The more electronegative atom draws electron density closer to itself • The more electronegative atom develops a partial negative charge (δ-) and the less electronegative atom develops a partial positive charge (δ+) • A bond which is polarized is a dipole and has a dipole moment • The direction of the dipole can be indicated by a dipole arrow
– The arrow head is the negative end of a dipole, the crossed end is the positive end

• Writing Lewis Structures
• Atoms bond by using their valence electrons • The number of valence electrons is equal to the group number of the atom
– – – – Carbon is in group 4A and has 4 valence electrons Hydrogen is in group 1A and has 1 valence electron Oxygen is in group 6A and has 6 valence electrons Nitrogen is in group 5A and has 5 valence electrons

• To construct molecules the atoms are assembled with the correct number of valence electrons

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•If the molecule is an ion, electrons are added or subtracted to give it the proper charge •The structure is written to satisfy the octet rule for each atom and to give the correct charge •If necessary, multiple bonds can be used to satisfy the octet rule for each atom

– Example
• Write the