Shapes of Organic Molecules; σ & π Bonds (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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Define straight-chain organic molecule.
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Define straight-chain organic molecule.
A straight-chain organic molecule is one in which all carbon atoms are connected to each other in one continuous, unbranched chain.
What distinguishes a branched organic molecule from a straight-chain molecule?
A branched organic molecule has one or more side groups (alkyl groups) attached to the main carbon chain, whereas a straight-chain molecule has no side branches.
True or False?
Cyclic organic compounds are always aromatic.
False.
Cyclic organic molecules simply have carbon atoms connected in a ring. They are aliphatic unless they contain a benzene ring, which would make them aromatic.
Define cyclic organic molecule.
A cyclic organic molecule is one in which the carbon atoms are connected to each other in a ring structure.
Straight, branched and cyclic organic molecules are all called .......... compounds, provided they do not contain a .......... ring.
Straight, branched and cyclic organic molecules are all called aliphatic compounds, provided they do not contain a benzene ring.
Give one example each of a straight-chain, a branched and a cyclic alkane.
Straight-chain: butane (C4H10). Branched: 2-methylbutane (C5H12). Cyclic: cyclopentane (C5H10).
True or False?
2-methylbutane is a straight-chain organic molecule.
False.
2-methylbutane is a branched molecule: it has a four-carbon main chain with a methyl group attached at carbon 2.
In a branched molecule, the side groups are attached to the .......... chain. In a cyclic molecule, carbons are connected in a .......... shape.
In a branched molecule, the side groups are attached to the main chain. In a cyclic molecule, carbons are connected in a ring shape.
What three structural arrangements can the carbon atoms take in aliphatic organic compounds?
Carbon atoms in aliphatic compounds can be arranged as straight chains, branched chains or cyclic rings (provided no benzene ring is present).
Define sigma (σ) bond.
A sigma bond is formed by the end-on overlap of atomic orbitals. Electron density is symmetrical about the line joining the two nuclei.
What hybridisation state does a carbon atom have when it forms four sigma bonds, and what shape does this produce?
A carbon forming four σ bonds is sp3 hybridised, giving a tetrahedral arrangement with bond angles of 109.5°.
True or False?
A double bond consists of one sigma bond and one pi bond.
True.
A double bond is made up of one σ bond (end-on orbital overlap) and one π bond (sideways p orbital overlap).
Define pi (π) bond.
A pi bond is formed by the sideways overlap of p orbitals. The two electron clouds lie above and below (or in front of and behind) the plane of the bonded atoms.
An sp2 hybridised carbon forms .......... sigma bonds and has .......... p orbital(s) left for sideways overlap, resulting in .......... pi bond(s) per carbon–carbon double bond.
An sp2 hybridised carbon forms three sigma bonds and has one p orbital(s) left for sideways overlap, resulting in one pi bond(s) per carbon–carbon double bond.
What hybridisation state is present in ethyne and what bond angle does it produce?
The carbon atoms in ethyne are sp hybridised, giving a linear arrangement with bond angles of 180°.
True or False?
A triple bond consists of one sigma bond and two pi bonds.
True.
A triple bond contains one σ bond and two π bonds, formed from the end-on overlap of one orbital pair and the sideways overlap of two pairs of p orbitals.
sp2 hybridised carbon atoms adopt a .......... arrangement with bond angles of ........... sp3 hybridised carbon atoms adopt a .......... arrangement with bond angles of 109.5°.
sp2 hybridised carbon atoms adopt a planar arrangement with bond angles of 120°. sp3 hybridised carbon atoms adopt a tetrahedral arrangement with bond angles of 109.5°.
Why does bond strength increase from single to double to triple bonds?
Each additional bond increases the electron density between the carbon atoms, making the bond stronger and harder to break. Bond strength order: single < double < triple.
Define planar molecule.
A planar molecule is one in which all atoms lie in the same plane.
Why does the presence of an sp2 hybridised carbon force a molecule to adopt a trigonal planar shape?
An sp2 carbon has three σ bonds that spread into a trigonal planar arrangement (120° bond angles) to minimise repulsion between bonding pairs of electrons.
True or False?
Ethene is a planar molecule.
True.
Ethene has two sp2 hybridised carbons, each forming a trigonal planar centre. All six atoms lie in the same plane.
sp2 hybridised carbon gives a .......... planar shape with bond angles of ........... sp hybridised carbon gives a .......... shape with bond angles of 180°.
sp2 hybridised carbon gives a trigonal planar shape with bond angles of 120°. sp hybridised carbon gives a linear shape with bond angles of 180°.
Propene has one sp3 centre and two sp2 centres. Which part of the molecule is planar and which is not?
The two carbons of the C=C double bond and all atoms directly bonded to them are in the same plane (trigonal planar centres). The CH3 group is tetrahedral and not part of the planar region.
True or False?
Ethyne is a non-planar molecule.
False.
Ethyne is a linear (and therefore planar) molecule. Its two sp hybridised carbons arrange their σ bonds at 180° to minimise repulsion.
Propyne has one .......... centre (from the C–C triple bond region) and one .......... centre (from the CH3 group).
Propyne has one linear centre (from the C–C triple bond region) and one tetrahedral centre (from the CH3 group).
Define trigonal planar arrangement.
A trigonal planar arrangement is a molecular geometry in which three groups around a central atom lie in the same plane at bond angles of 120°, as seen in sp2 hybridised carbons.
Give two examples of planar molecular shapes and state a bond angle for each.
Examples: trigonal planar (120°, e.g. ethene) and linear (180°, e.g. ethyne). Both have all atoms in the same plane.
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