Reactions of Organic Substances on Primitive Earth

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Reactions of Organic Substances on Primitive Earth
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Organic Chemistry

• Organic chemistry is the study of the compounds
  of carbon with emphasis on those containing C-H
  bonds
• Of the 25 million known substances, over 80 are
  classified as being organic compounds

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Contd

• The large number of carbon compounds is due to
  ability of C to form stable bonds with other C
  atoms (called catenation), ability of C to form
  single, double, and triple bonds and its ability
  to form stable bonds with H, Cl, O, N, S
• Simplest organic substances are compounds of C
  and H (hydrocarbons)

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C6H6
CnH2n2
CnH2n
CnH2n-2
CnH2n
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Isomers

• Isomers are different substances that have the
  same molecular formula (operational definition)
• Isomers are substances with the same atoms, but
  different bonds (theoretical definition)

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Isomers of C4H10
The more atoms in a molecule, the more different
ways those atoms can be arranged, e.g., the atoms
in C20H42 can be arranged in 366,319 different
ways or C30H62 in 4,111,846,763 ways!
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Geometries of Molecules

• Carbon is able to form single, double or triple
  bonds with like atoms
• Each of these bonds has a different geometry
  tetrahedral for single bonds, trigonal planar for
  double bonds, and linear for triple bond
• Variations in geometries at C to C bonds extend
  the range of complex molecules that carbon can
  form

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Functional Groups

• Although alkanes are relatively unreactive, it is
  possible to replace some hydrogen atoms with more
  electronegative atoms such as Cl, O, or N which
  become reactive sites
• Reactive sites in an organic molecule are called
  functional groups

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Functional Groups
One H of C2H6 can be replaced by a Cl, by an OH,
or by an NH2 group. These groups become the
reactive sites of the molecule while the
hydrocarbon part of the molecule (C2H5) remains
unreactive
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Families of Organic Compounds
Families of organic compounds all can have the
same R groups but each has a different functional
group
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R is called an alkyl radical - stands for the
unreactive hydrocarbon part of the molecule such
as CH3- or C2H5-
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Functional Groups and Physical Properties

• Functional groups change both chemical and
  physical properties
• Presence of electronegative atoms changes bond
  and molecular polarities which in turn change
  solubilities
• Intermolecular forces are greatly changed by
  presence of OH or NH groups which make H-bonding
  possible

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Energy Requirements for Organic Reactions

• All chemical reactions are accompanied by energy
  changes usually as heat
• Exothermic reaction - reaction, which when
  initiated, produces heat and sustains itself
• Endothermic reaction- reaction, which when
  initiated, requires continuous input of heat to
  sustain the reaction

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Thermochemical Equations (Symbolic world)

• Modification of a chemical equation that includes
  states of participants and heat change that
  accompanies the reaction
• States specified by (s) solid, (g) gas, (l)
  liquid, and (aq) solute in water
• Heat change for number of moles specified by
  equation indicated as DH lt 0 for exo and DH gt 0
  for endothermic

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Continued...

• 2 H2(g) O2(g) gt 2 H2O(g) DH -116 kcal
• 2 moles of H2(g) react with 1 mole of O2(g) to
  produce 2 moles of H2O(g) while producing 116
  kcal of heat (Exo)
• 2 HgO(s) gt 2 Hg(l) O2 (g) DH 43 kcal
• 2 moles of HgO(s) produces 2 moles of Hg(l) and 1
  mole of O2(g) while consuming 43 kcal of heat
  (Endo)

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Energy Change and Energy Requirement for a
Reaction

• In a chemical reaction involving covalent
  bonding, energy must be supplied (endothermic) to
  break bonds in reactants and energy is produced
  (exothermic) when bonds are formed in the
  products
• The difference between energy supplied to break
  bonds (endo) and energy produced when bonds form
  (exo) is DH

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Continued...

• Bond energy (BE) is the energy required to break
  a mole of bonds between two atoms
• Values can be obtained from standard tables
• Endo - Exo DH
• Sum of BE of all bonds in reactants - Sum of BE
  of all bonds in products DH

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Continued...

• For 2 H2(g) O2(g) gt 2 H2O(g) DH 2
  BEH-H BEOO - 4 BEO-H
  2(104) 119 - 4(111) -117 kcal
• The reaction is exothermic and the heat produced
  when molar quantities specified by balancing
  coefficients react is estimated to be 117 kcal

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Heat Required to Initiate a Reaction

• The minimum energy required to initiate a
  reaction is called the energy of activation, Ea
• The energy of activation can be obtained from the
  energy profile of a reaction which is a graph of
  energy vs. progress of reaction

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Energy Profile of a Reaction
Ea
The energy change for a reaction is DH, while
the energy needed to initiate a reaction is Ea
http//gcsechemistry.com/rc1.htm
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Sources of Energy Available for Chemical
Reactions on Primitive Earth

• Nuclear radiation
• Solar radiation
• Cosmic radiation
• Lightning
• Thermal energy from volcanoes

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Nuclear Radiation

• Many of the atoms produced during a supernova are
  unstable and decay by alpha or beta radiation to
  produce more stable nuclei and heat
• Ionizing radiation can break bonds, produce free
  radicals and heat and thus can cause chemical
  reactions

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Solar Radiation

• Surface of the sun is about 6000 K
• All matter having T gt 0 K, radiate
  electromagnetic radiation
• Electromagnetic radiation is energy moving
  through space in the form of a wave having the
  following properties

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Electromagnetic Radiation
Wavelength,?, is distance between consecutive
troughs or crests of a wave
n
Frequency, ?, is the number of wave-lengths
passing a point in one second
n
n
Silberberg"Chemistry", Mosby, NY, 1996, 257
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Relations for Electromagnetic Radiation

• c ln 3 x 108 m/s speed of light (or any
  form of electromagnetic radiation)
• Energy, E å n or E å 1/l

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Electromagnetic Spectrum
Visible (VIS)
Tro, 162
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Prism Bending Rays of White Light
Short wavelengths bend more than long wavelengths
giving rainbow ROYGBIV
Brescia et al, General Chemistry, 5th Saunders,
1988, 140f
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Visible Spectrum (VIS)
Tro, 162
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Energy Distribution in Solar Radiation
UV radiation can break chemical bonds, produce
free radicals, and thus can cause chemical
reactions
Schwartz et al, Chemistry in Context,Brown, IA,
1994, 37
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Cosmic Radiation

• Particles in or near a supernova are accelerated
  to tremendous velocities and make up cosmic
  radiation
• When they collide with molecules, cosmic rays can
  break chemical bonds and create free radicals,
  and thus can cause chemical reactions

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Other Energy Sources

• Electrical energy associated with lightning can
  easily break chemical bonds and produce free
  radicals and thus can cause chemical reactions
• Volcanoes are sources of thermal energy which can
  increase the rates of reactions

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Chemical Evolution

• Oparin Hypothesis (1924)
• Biological evolution was preceded by chemical
  evolution

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Time Frame for Chemical Evolution on Earth

• Earth is estimated by ratios of Pb to U to be
  about 4.5 to 4.8 billion years old
• Life is estimated from fossil records to have
  begun about 3.3 billion years ago
• Chemical evolution of precursors to life thus
  occurred over 1.2-1.5 billion years

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Conditions for Precursor Reactions

• Earliest atmosphere contained H2, so major
  components were H2O, CH4 , NH3 and N2
• Assume average temperature was about same as
  today, 288 K
• Sources of energy were cosmic rays, nuclear
  radiation, UV light, lightning, and heat
  (volcanoes and nuclear decay)

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Apparatus for testing Oparin's Hypothesis -
produces bioorganic compounds from a simulated
prim-itive atmosphere using spark discharges to
simulate lightning (S. Miller- 1955)
Miller, Livingin Environment, 12th, Thompson, CA,
2002, 104
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First Reactions

• Formation of HCN
  CH4 NH3 gt HCN 3 H2
• Formation of formaldehyde CH4
  H2O gt CH2O 2 H2
• These substances serve as precursors to important
  biological molecules

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Formation of Molecules Associated with Life

• Formation of carbohydrates (sugars)
  n(CH2O) gt carbohydrates
• Formation of DNA bases n(HCN) gt
  bases for DNA
• Formation of amino acids (components of proteins)
  
  CH4 HCN CH2O NH3 gt amino acids

Details of these reactions are given in later
lectures
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Summary of Chemical Evolution
Fox/Dose, Molecular Evolution and the Origin of
Life, rev ed, Dekker, NY,1977, 67
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Fox/Dose, Molecular Evolution and the Origin of
Life, rev ed, Dekker, NY,1977, 67