CHEM1201 Lecture Notes - Lecture 14: Polyethylene, Substituent, Benzene
7 Jun 2018
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Homologous series: a series of compounds with the same general formula, usually varying
by a single parameter such as the length of a carbon chain
• Alkanes CnH2n+2 • Alkenes CnH2n
• Alkynes CnH2n–2
Structural formulas:
- A structural formula shows not only the numbers of each kind of atoms but also how
the atoms are bonded.
- Condensed structural formula groups the hydrogen atoms with the carbon atom to
which they are bonded.
-
- Carbon skeleton formula, also called a line formula, shows the carbon–carbon bonds
onlyas lines.
-
- Structural formulas are two-dimensional while space-filling or ball-and-stick models
are three dimensional.
Valence Bond Theory:
- Valence Bond theory (VB) approaches chemical bonding based on an extension of
the quantum-mechanical model (more accurate than VSEPR)
- when orbitals on atoms interact, they make a bond
- these orbitals can be hybridized atomic orbitals, a kind of combination of two or
more standard atomic orbitals
- when two atoms approach each other, the electrons and nucleus of one atom
interact with the electrons and nucleus of the other atom
- if the energy of the system is lowered because of the interaction, a chemical bond
forms
- a chemical bond results from the overlap of two half-filled orbitals with spin-pairing
of the two valence electrons (or less commonly the overlap of a completely filled
orbital with an empty orbital)
- the geometry of the overlapping orbitals determines the shape of the molecule
o bonding in H2 (one sigma bond)
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Types of bonds:
- a sigma () bond:
o results when the interacting atomic orbitals point along the axis connecting
the two bonding nuclei
o either standard atomic orbitals or hybrids
o s to s, p to p, hybrid to hybrid, s to hybrid, etc.
o
- a pi () bond:
o results when the bonding atomic orbitals are parallel to each other and
perpendicular to the axis connecting the two bonding nuclei
o between unhybridised parallel p orbitals
o
- the interaction between parallel orbitals is not as strong as between orbitals that
point at each other, i.e. bonds are STRONGER than bonds
Sigma bonds in methane:
- atom with four electron groups around it
o tetrahedral geometry
o 109.5o angles between hybrid orbitals
o four sigma bonds H(s)-C(sp3)
o
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Homologous series: a series of compounds with the same general formula, usually varying by a single parameter such as the length of a carbon chain: alkanes cnh2n+2 alkenes cnh2n alkynes cnh2n 2. A structural formula shows not only the numbers of each kind of atoms but also how the atoms are bonded. Condensed structural formula groups the hydrogen atoms with the carbon atom to which they are bonded. Carbon skeleton formula, also called a line formula, shows the carbon carbon bonds only as lines. Structural formulas are two-dimensional while space-filling or ball-and-stick models are three dimensional. Valence bond theory (vb) approaches chemical bonding based on an extension of the quantum-mechanical model (more accurate than vsepr) When orbitals on atoms interact, they make a bond these orbitals can be hybridized atomic orbitals, a kind of combination of two or more standard atomic orbitals.
