CHEM10007 Lecture Notes - Lecture 32: Trigonal Planar Molecular Geometry, Acetylene, Homologous Series

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azurealpaca671

12 Jun 2018

School

University of Melbourne

Department

Chemistry

Course

CHEM10007

Professor

Andrew Drinnan

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LECTURE 32

ALKYNES & ORGANIC FUNCTIONAL GROUPS (1)

CHAPTER 16 - BIOCHEMISTRY

ALKYNES

•Alkynes are a homologous series of hydrocarbons having the general molecular formula

CnH2n-2 where n = 2,3,4, etc.

•Eg. C2H2, C3H4, C4H6, C5H8, etc.

•Alkynes are characterized by the presence of a triple bond between two carbon atoms the bond is

much stronger and is non rotatable.

•Nomenclature – analogous to alkenes, however add the suffix ‘yne’ to the parent alkane name

(e.g. ethyne)

•Eg. Ethyne (C2H2)

STRUCTURE & BONDING IN ALKYNES

•The two carbons of a Carbon-Carbon triple bond have a linear planar geometry (180 degrees)

and each carbon is dicoordinate (digonal).

•Bond length of C≡C is 1.2 Å, bond strength is 837 kJ/mol.

BONDING DESCRIPTION OF ETHYNE

•Each carbon atom uses an s and a p orbital, which are blended together to form sp hybrids which

point in opposite directions. Overlap between sp

hybrid orbitals on each carbon results in the

formation of a single strong σ-bond between

each carbon. The remaining two p orbitals on

each carbon then form two π bonds (in

orthogonal planes). This leads to a linear

structure for the system. (see diagrams below).

•A combination of one sp-sp (σ) and two p-p

(π) overlap results in sharing of 6 electrons and

formation of C-C triple bond (C≡C)

•The remaining 2sp orbitals need only 2H to

form a (σ) bond.

•The structure provides ethene a stronger and

shorter C-C bond, a H-C and C-C angle of

180°and ensures the molecule is linear and not

tetrahedral or trigonal planar.

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Document Summary

Alkynes: alkynes are a homologous series of hydrocarbons having the general molecular formula. Structure & bonding in alkynes: the two carbons of a carbon-carbon triple bond have a linear planar geometry (180 degrees) and each carbon is dicoordinate (digonal), bond length of c c is 1. 2 , bond strength is 837 kj/mol. Bonding description of ethyne: each carbon atom uses an s and a p orbital, which are blended together to form sp hybrids which point in opposite directions. Overlap between sp hybrid orbitals on each carbon results in the formation of a single strong -bond between each carbon. The remaining two p orbitals on each carbon then form two bonds (in orthogonal planes). 180 and ensures the molecule is linear and not tetrahedral or trigonal planar. Nomenclature of alkynes: the nomenclature of alkynes is analogous to that of alkenes, however we add the suffix yne" to the parent alkane name (eg.

Related Questions

For each of the following, fill in the blank with the correct response. All of these fill-in-the-blank problems pertain to material covered in the sections on alkanes, alkenes and alkynes, aromatic hydrocarbons, and hydrocarbon derivatives.

a. The first "organic" compound to be synthesized in the laboratory, rather than being isolated from nature, was _ _ _ _ _, which was prepared from _ _ _ _ _.

b. An organic compound whose carbon-carbon bonds are all single bonds is said to be _ _ _ _ _.

c. The general orientation of the four pairs of electrons around the carbon atoms in alkanes is _ _ _ _ _.

d. Alkanes in which the carbon atoms form a single unbranched chain are said to be _ _ _ _ _ alkanes.

e. Structural isomerism occurs when two molecules have the same number of each type of atom but exhibit a different arrangement of the _ _ _ _ _ between those atoms.

f. The systematic names of all saturated hydrocarbons have the ending _ _ _ _ _ added to a root name that indicates the number of carbon atoms in the molecule.

g. For branched hydrocarbon, the root name for the hydrocarbon comes from the number of carbon atoms in the _ _ _ _ _ continuous chain in the molecule.

h. The positions of substituents along the hydrocarbon framework of a molecule are indicated by the _ _ _ _ _ of the carbon atom to which the substituents are attached.

i. The major use of alkanes has been in _ _ _ _ _ reactions, as a source of heat and light.

j. With very reactive agents, such as the halogen elements, alkanes undergo _ _ _ _ _ reactions, whereby a new atom replaces one or more hydrogen atoms of the alkane.

k. Alkenes and alkynes are characterized by their ability to undergo rapid, complete _ _ _ _ _ reactions, by which other atoms attach themselves to the carbon atoms of the double or triple bond.

l. Unsaturated fats may be converted to saturated fats by the process of _ _ _ _ _.

m. Benzene is the parent member of the group of hydrocarbons called _ _ _ _ _ hydrocarbons.

n. An atom or group of atoms that imparts new and characteristic properties to an organic molecule is called a _ _ _ _ _ group.

o. A _ _ _ _ _ alcohol is one in which there is only one hydrocarbon group attached to the carbon atom holding the hydroxyl group.

p. The simplest alcohol, methanol, prepared industrially by the hydrogenation of _ _ _ _ _.

q. Ethanol is commonly prepared by the _ _ _ _ _ of certain sugars by yeast.

r. Both aldehydes and ketones contain the _ _ _ _ _ group, but they differ in where this group occurs along the hydrocarbon chain.

s. Aldehydes and ketones can be prepared by _ _ _ _ _ of the corresponding alcohol.

t. Organic acids, which contain the _ _ _ _ _ group, are typically weak acids.

u. The typical sweet-smelling compounds called _ _ _ _ _ result from the condensation reaction of organic acid with an _ _ _ _ _.

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CHEM10007 Lecture Notes - Lecture 32: Trigonal Planar Molecular Geometry, Acetylene, Homologous Series

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