Monohalo alkanes are condensed with aliphatic mono olefines in the presence of a metal halide catalyst of the Friedel Crafts type, whereby monohalo alkanes having a molecular weight equal to the sum of the initial monohalo alkane and at least one aliphatic olefine constitute the principal product of the reaction. In a specific embodiment, monohalo alkanes having at least five carbon atoms are produced by catalytically condensing a monohalo alkane having at least three carbon atoms with an olefine. The higher boiling monohalo alkanes so formed can react with a further molar proportion of olefine to form still higher boiling monohalo alkanes. The olefines employed are the normally gaseous or liquid olefines including ethylene, propylene, butylenes and higher normally liquid olefines, the latter including various polymers of normally gaseous olefines. The initial monohalo alkanes may be primary, secondary or tertiary. Monochloro alkanes and monobromo alkanes are preferred as starting materials, but the corresponding iodo- and fluoro-alkanes may be used. The Friedel Crafts type metal halide catalysts include aluminium chloride and bromide, ferric chloride, bismuth chloride, zinc chloride and zirconium chloride. These catalysts may be employed as such, composited one with another, or supported on solid carriers such as alumina, activated charcoal, crushed porcelain, raw and acid treated clays, diatomaceous earth, pumice and firebrick. The condensing reaction consists apparently in the addition of the monohalo alkane to the double bond of the olefine; for example, tertiary butyl chloride, with ethylene yields 4-chloro-2,2-dimethyl butane, and with propylene and n-butylene, mono-chloro-heptane and mono-chloro-octane respectively, whilst tertiary butyl bromide with propylene gives monobromoheptane. The condensation is generally carried out while maintaining a pressure sufficient to keep at least a substantial proportion of the reactants in the liquid state. When condensing monohalo alkanes, particularly tertiary butyl chloride, in the presence of bismuth chloride, with olefines containing at least three carbon atoms such as propylene, butylenes and amylenes, a temperature between 0 DEG to 50 DEG C. is utilizable, and for the reaction between tertiary butyl chloride and ethylene a temperature of 50 DEG to 125 DEG C. is suitable. With ferric chloride or zirconium chloride as catalyst, the temperature may be from 0 DEG to 50 DEG C.; aluminium chloride requires a lower temperature. An inert solvent may be used, such as a paraffinic hydrocarbon, e.g. n-pentane, or a nitroparaffin such as nitromethane. Organic peroxides, for example, benzoyl peroxide and ascaridole, may be present. The monohalo alkanes so produced may be hydrogenated or dehydrohalogenated and then hydrogenated to give hydrocarbons of high anti-knock value. The monohalo alkanes employed as starting materials may be prepared by the addition of a hydrogen halide to an olefine, generally in the presence of a Friedel Craft type metal halide catalyst or an acid such as sulphuric or phosphoric acid; non-tertiary olefines having at least 3 carbon atoms give rise to secondary monohalo alkanes, whilst tertiary olefines such as isobutene and trimethyl ethylene form tertiary halo alkanes; primary monohalo alkanes may be prepared by the addition of hydrogen halide to ethylene, or by reacting a primary alcohol with hydrogen halide in the presence of zinc chloride; primary monobromo alkanes are prepared by the addition of hydrogen bromide to a 1-alkene in the presence of peroxides or sunlight. The olefines employed may be prepared from the products of catalytic or thermal cracking of oils, by dehydrogenation of paraffinic hydrocarbons or by dehydrating alcohols. In examples: (1) tertiary butyl chloride is reacted with ethylene in the presence of bismuth chloride or ferric chloride and the reaction products fractionated giving 4 - chloro - 2,2 - dimethyl butane; (2) tertiary butyl chloride and propylene are condensed in the presence of bismuth chloride or zirconium chloride and 4-chloro-2,2-dimethyl pentane obtained by fractionation of the reaction products; (3) tertiary butyl chloride and normal butylene (mainly 2-butene with some 1-butene) are condensed in the presence of bismuth chloride to give mono-chloro octane as the principal product with some octene; (4) tertiary butyl bromide and propylene are reacted in the presence of bismuth chloride and ascaridole to yield mono-bromo heptane and higher boiling materials; (5) isopropyl chloride and ethylene are reacted in the presence of ferric chloride to yield a monochloroheptane presumably formed from one mole of the isopropyl chloride and two moles of ethylene. The Specification as open to inspection under Sect. 91 comprises also the use of arylalkenes as starting materials. This subject-matter does not appear in the Specification as accepted.