DF 9 - what do the molecules look like Flashcards
specification reference - (c) (s) (t)
what do chemists use to explain how molecules behave
models of molecular structures.
why is representing structures in a two dimensional way on paper a bad thing
it can give a misleading picture of what the molecules look like.
what do pairs of electrons in covalent bond do
they repel one another so they arrange themselves around the central atom as far as possible to minimise repulsion
how are the C-H bonds arranged in methane
they are directed so they point towards the corners of a regular tetrahedron. the carbon atom is at the centre of the tetrahedron
what is the bond angle of H-C-H bonds in methane
109.5°
what does a normal line show
the bond is in the plane of the paper
what does a dotted line show
the bond is in a direction behind the plane of the paper
what does a wedged line show
the bond is in a direction in front of the plane of the paper
what is the electron pair repulsion theory
the shape of a molecule or ion is determined by the number of electron pairs in the outer shell surrounding the central atom. as electrons all have a negative charge, each electron pair repels other electron pairs. the electron pairs push one another as far apart as possible
which is more electron dense - a lone pair or bonded pair
a lone pair. this means it repels more than a bonded pair
what is the shape and bond angle of a molecule with 2 areas of electron density
linear - 180°
what is the shape and bond angle of a molecule with 3 areas of electron density
trigonal planar - 120°
what is the shape and bond angle of a molecule with 4 areas of electron density
tetrahedral - 109.5°
what is the shape and bond angle of a molecule with 5 areas of electron density
trigonal bipyramid - 120° and 90°
what is the shape and bond angle of a molecule with 6 areas of electron density
octahedral - 90°
what are the bond angles of hydrocarbons
109.5°
are hydrocarbon chains straight
no they are a zigzag of carbon atoms
what is the shape of a hydrocarbon chain often represented by
a skeletal formula
what does skeletal formula show
they only show the carbon skeleton and associating functional groups of the molecule. they do not assist in visualising shapes of molecules
what is an isomer
molecules which have the smae molecular formula but differ in the way their atoms are arranged. they have different physical and chemical properties
why is isomerism common in carbon compounds
because of the variety of ways in which carbon atoms can form chains and rings
what are the two ways in which atoms can be arranged differently in isomers
structural isomers and stereoisomers
what is a structural isomer
the atoms are bonded together in a different order in each isomer
what is a stereoisomer
the order of the bonding in the isomers is the same but the arrangement of the atoms in space is different in each isomer
which isomers have the same molecular formula but different structural arrangement of atoms
structural isomers
what are the three types of structural isomerism
chain, position and functional group isomerism
what is a chain isomer
two or more compounds with similar chemical formulas but distinct carbon atom arrangements in straight or branched chains
when does chain isomerism occur
in methane, ethane, and propane only one alkane corresponds to the molecular formula
with four or more carbon atoms in a chain different arrangements are possible.
give two structural chain isomers
butane and 2-methylpropane have the molecular formula C4H10
what does the different structure in the chain isomers lead to
different properties
as the number of carbon atoms increases..
.. the number of possible chain isomers increases
what is positonal isomerism
occurs when a functional group can occupy different positions on the same carbon chain
when does positional isomerism occur
when there is an atom, or groups of atoms substituted in a carbon chain or ring, these are functional groups.
give an example of positional isomers
propan-1-ol and propan-2-ol
the functional group is OH and is situated at different places on the hydrocarbon chains
what is functional group isomerism
when substances have the same molecular formula but different functional groups.
what does it mean if compounds have different functional groups
they belong to a different homologous series
give an example of functional group isomers
propan-1-ol and methoxyethane have the same molecular formula (C3H8O) but different functional groups
propan-1-ol has the OH functional group (alchohol)
methoxyethane has the -O functional group (ether)
they have different physical and chemical properties due to the different functional groups
which isomers have the same structural formula but differ in the way their atoms are arranged in space
stereoisomers
what are the two types of stereoisomerism
E/Z or cis/trans isomerism
optical isomerism
name a compound with two E/Z isomers
but-2-ene
show the isomers of but-2-ene
H CH3 H H
\ / \ /
C=C C=C
/ \ / \
CH3 H CH3 CH3
how would we turn make both methyl side chains situate on the same side of the C=C bond in but-2-ene
you would have to spin one end of the molecule round in relation to the other end. this can only be done by breaking the 𝛑 bond in the double bond
what is the average bond enthalpy to break the 𝛑 bond in but-2-ene
+270kJmol-1
—> not available at room temperature
what bond has to be broken to interconvert the forms of but-2-ene
covalent has to be broken and then another reformed
what is the form of but-2-ene with the same groups on the same side of the double bond
cis-but-2-ene or (Z)-but-2-ene
what is the form of but-2-ene with the same groups on opposite sides of the double bond
trans-but-2-ene or (E)-but-2-ene
when does the cis/trans rule work
when either of the two groups of either side of the double bond are the same or if there is a hydrogen on both carbon atoms
give an example of where the cis/trans naming system cannot be used
naming haloalkanes
cis (Z isomer)
trans (E isomer)
why do E and Z isomers have different properties
they are different compounds
we use the cis/trans naming system when…
…one of the groups on each carbon of C=C double bond is the same
we use the E/Z naming system for..
..compounds that have an H on each carbon of C=C
why is there a lack of free rotation around the C=C double bond
Due to the pi-bond formation, rotation is restricted because if we rotate the atoms around pi bond, then the overlapping of the p orbitals will get disturbed.
